Development of enzymatic biodiesel production at commercial scale is dependent on the reactor systems. Various reactors, including batch reactors, packed-bed reactors and supercritical reactors, are studied for bio­diesel production. Most of the studies have done on batch reactors and packed-bed reactors. Batch reactors are simple to use in the laboratory. But shear stress caused by stirrer would disrupt the enzyme life (Tan et al., 2010). Batch operation is a laborious process and is not suitable for automation (Chen et al., 2010). Packed-bed reactors are continuous and are a good alter­native for batch reactors to lower the shear stresses (long-term enzyme stability) and to make the process economical (Wang et al., 2010). In addition, this system offers high bed volume and is simple to scale up (Hama et al., 2011). Because of its continuous mode, stepwise addition of alcohol is possible in order to reduce the inactivation of the enzyme caused by excess alcohol. Lipase inhibition due to the cloggage by glyc­erol accumulation inside the reactor is a major challenge (Xu et al., 2012). This can be resolved using more than one column in the reactor. Yoshida et al. (2012) devel­oped a reactor in which a reactant solution is pumped through a column containing immobilized recombinant

Aspergillus oryzae and the effluent from the column is recycled into the same column with a stepwise addition of methanol. This reactor system gave better lipase activ­ity up to five cycles with 96.1% FAME content.

Wang et al. (2011b) developed a four-packed-bed reactor in order to provide longer residence time to the reaction mixture in the reactor and to lower lipase inhi­bition by product accumulation. A single-packed-bed reactor and the four-packed-bed reactor were used to produce biodiesel by using refined soybean oil with

P. cepacia lipase. Over 88% conversion rate and great sta­bility were achieved with the four-packed-bed reactor compared to single-packed-bed reactor (Wang et al.,

2010) . This process improved the reaction efficiency and additionally, the cost of biodiesel production can be reduced by effective recycling of enzyme (Fjerbaek et al., 2009).

Supercritical reactors are also investigated for enzy­matic biodiesel production. D. Oliveira and J. V. Oli­veira (2001) produced biodiesel from palm kernel oil in the presence of Novozym 435 and Lipozyme IM in supercritical carbon dioxide. Lipozyme IM showed bet­ter conversion (77.5%). But the problem is high pres­sure (beyond 200 bar) used in this process. Study by Taher et al. (2011) has given only 49.2% conversion rate with lamb-meat fat in supercritical carbon dioxide by Novozym 435. Supercritical reactors could not be commercialized due to low conversion rate and high cost of the system. Subsequently, a technically improved packed-bed reactor system with high trans­esterification efficiency is a good alternative for indus­trial scale-up of enzymatic biodiesel production in an economic way.

CONCLUSIONS

Energy crisis and environmental concerns raised the necessity for the new biofuels. Biodiesel is a clean alter­native to fossil fuel. A green approach for biodiesel pro­duction through enzymatic biodiesel production has gained a lot of attention due to the drawbacks of chem­ical methods. Promising enzymatic processes are estab­lished for biodiesel production. The main obstacle for the industrialization of enzymatic process would be overall cost of production. Production cost could be reduced by increasing the productivity or by increasing the catalytic efficiency of lipases. Immobilization and ge­netic engineering methods appear to be an attractive way to obtain more active, stable, and reusable lipases in different reaction systems. Operational parameters like water content, temperature, solvent, acyl acceptors, and so on plays key role in transesterification process. Along with all these technical operational conditions, novel bioreactor designing has also promising challenges in order to make biodiesel a great potential commercial fuel in future.

Bioelectrochemistry of Microbial Fuel Cells
and their Potential Applications in Bioenergy

Minghua Zhou1’ , Jie Yang1, Hongyu Wang1, Tao Jin1,

Daniel J. Hassett2’ Tingyue Gu3’

^Key Laboratory of Pollution Processes and Environmental Criteria (Ministry of Education), College of Environmental
Science and Engineering, Nankai University, Tianjin, China, ^Department of Molecular Genetics,
Biochemistry and Microbiology, University of Cincinnati, College of Medicine, Cincinnati, OH, USA,
^Department of Chemical and Biomolecular Engineering, Ohio University, Athens, OH, USA
*Corresponding author email: gu@ohio. edu;zhoumh@nankai. edu. cn

OUTLINE

Introduction 132

Bioelectrochemistry of MFC 132

Electrode Reactions in MFC 132

Anode Reaction 132

Cathode Reaction 133

Electron Transfer Methods 133

DET for Anodic Biofilms 134

MET for Anodic Biofilms 134

Electrogens in Biofilms for MFCs 135

Biocathodes 136

Electron Transfer for Biocathodes 137

DET for Biocathodes 137

MET for Biocathodes 137

Biofilm Electrochemistry for Enhanced MFC

Performance: A Molecular Biology Perspective 139

Bacterial Metabolism: How to Power MFCs through Respiratory/Anaerobic Fluxes 139

Mediator-Less Factors Affecting MFC Performance 139 TFP (or “Nanowires”): Geobacter and Shewanella Species as Model Organisms 139

Cytochromes (Cell-Bound) 140

Brief Synopsis of the S. oneidensis MR-1 Bioelectrochemical Machinery in Reverse:

Potential Role in the Biosynthesis of Biofuels in MFCs 142

Mediators for Accelerated Electron Transfer in Biofilms 142

Flavins 142

Phenazines 143

MFCs for Wastewater Treatment with Concomitant Electricity Production 143

MFC Reactor Designs 143

Substrates Used in MFCs 145

Simple Biodegradable Organics 145

Wastewater Types 146

Lignocellulosic Biomass 146

Summary and Perspectives 147

References 147

Bioenergy Research: Advances and Applications

http://dx. doi. org/10.1016/B978-0-444-59561-4.00009-7

INTRODUCTION

Currently, the energy sources utilized in our society are mainly fossil fuels such as oil, natural gas and coal (Makarieva et al., 2008). However, their supplies are limited and nonrenewable (Logan, 2009). When fossil fuels are combusted, their carbon, sulfur and nitrogen contents are converted into carbon oxides, sulfur oxides and nitric oxides, respectively, resulting in greenhouse gas emission and environmental pollution (e. g. acid rain). With dwindling oil reserves, global warming signs and worsening air pollution in many countries, more efforts are devoted to the use of renewable energy such as solar, wind and bioenergy. Bioenergy is a sus­tainable alternative to fossil fuels as part of an integrated energy solution to alleviate the worldwide energy crisis and environmental pollution problems (Srikanth and Venkata, 2012).

Recently, microbial fuel cells (MFCs) have been inten­sively investigated in many academic labs as a potential technology for bioenergy production from organic car­bon sources such as wastewater, sludge and some ligno — cellulosic biomass (Allen and Bennetto, 1993; Lovley, 2006b; Rhoads et al., 2005). In a typical MFC, the microbes forming the anodic biofilm oxidize the sub­strates (organic material) by anaerobic respiration (Bond and Lovley, 2003; Logan et al., 2006) and release electrons (e“) and protons (H+) (Srikanth and Venkata, 2012). The electrons are transferred to the anode and then reach the cathode via an external circuit. Simulta­neously, protons in the solution travel through a proton exchange membrane (PEM) and reach the cathode where electrons are used to reduce oxygen. In this fashion, electricity is generated by converting the energy stored in the chemical bonds in the organic matter (or feedstock) provided to each system (Choi et al., 2003a; Gil et al., 2003; Huang et al., 2011b; Moon et al., 2006; Osman et al., 2010). Thus, MFCs produce bioelectricity directly instead of a biofuel in the process of degrading organic matter in the wastewater (Chaudhuri and Lov — ley, 2003; Oh and Logan, 2005; Park and Zeikus, 2000).

Bioelectricity production by an MFC was first reported by Petter (1911). Not much research was done on MFCs until 1980s when mediators were found to improve MFC power density greatly. However, externally supplied mediators such as methyl viologen, neutral red, and thio- nine are not sustainable. They are expensive and toxic, limiting their uses to academic research (Du et al., 2007). In recent years, some microorganisms such as Shewanella putrefaciens (Kim et al., 2002), Rhodoferax ferrireducens (Chaudhuri and Lovley, 2003), and Geobacteraceae sulfurre — ducens (Bond and Lovley, 2003) have been found to trans­fer electrons from the cytoplasm where metabolic respiration occurs to an external electrode surface (anode), resulting in the development of mediator-less MFCs.

Intensive research efforts from 1990 to 2010 have improved MFC power densities by several orders of magnitude to up to several watts per square meter (anode area) under optimal laboratory conditions. Recently, Tong et al. (2012) compared the power densities between MFCs and conventional fuel cells and found that MFCs were still behind by three orders of magni­tude. It is unrealistic to expect MFCs to catch up with chemical fuel cells because the latter uses pure hydrogen, ethanol or other high-energy-density fuels rather than wastes. However, it is still necessary to improve MFC power densities much further to make the MFC power generation practically useful. Major breakthroughs are needed in biofilm engineering, materials for electrodes and reactor configuration to achieve far better bio­electrochemical performance and to lower the currently rather high costs in MFC construction, maintenance and operation (Zhou et al., 2012). This chapter addresses various bioelectrochemical issues in MFC operation for the improvement of MFC performance.