Conditions for pretreatment of cotton stalks using P. chrysosporium by SSC have also been explored. While substrate moisture content significantly affects lignin degradation, supplementation with modified salts did not affect the reaction process. Over a period of 14 days, SSCat 75% moisture content without salts resulted in 27.6% lignin degradation, 71.1% solids recovery and 41.6% availability of carbohydrates, suggesting that mi­crobial pretreatment by SSC has the potential to be a low-cost, environmentally friendly alternative to chemi­cal approaches (Shi et al., 2008).

RICE STRAW

Fungal pretreatment of rice straw for improved enzy­matic saccharification has been reported. Yamagishi et al. (2011) tested 17 C. stercoreus isolates for their ability to treat rice straw for improved enzymatic hydrolysis. A negative correlation was found between cellulase and xylanase activity in these isolates and enzymatic saccharification yields in the pretreated straw. A 25-day pretreatment with the strain C. stercoreus TY-2 led to a more than fivefold increase in enzymatic saccharification yield compared to untreated control samples, suggesting this isolate has the potential for biological pretreatment of rice straw under conditions of low energy input. A 15-day pretreatment of rice straw with P. chrysosporium in an optimized media resulted in a treated biomass with an enzymatic digestibility of 64.9% of the theoretical maximum glucose yield. When the fungal-pretreated rice straw was used as a substrate in simultaneous saccharifi­cation and fermentation (SSF), a 9.49 g/l ethanol concen­tration, 58.2% of the theoretical maximum production yield, and 0.40 g/l/h productivity were achieved after 24 h and a 62.7% of the theoretical maximum ethanol yield was expected after 96 h (Bak et al., 2009).

When rice straw was pretreated with the wood-rot fun­gus, Dichomitus squalens, for 15 days, an enzymatic digest­ibility of 58.1% of theoretical glucose yield was reached for the treated biomass. When the pretreated rice straw was used as a substrate for ethanol production in SSF, the ethanol production yield and productivity were 54.2% of the theoretical maximum and 0.39 g/l/h, respectively, after 24 h (Bak et al., 2009). Taniguchia et al. (Taniguchi et al., 2005) reported the effect on rice straw composition and susceptibility to enzymatic hydrolysis after pretreat­ment with four white-rot fungi (P. chrysosporium, Trametes versicolor, C. subvermispora, and P. ostreatus). Among the four strains, P. ostreatus selectively degraded the lignin fraction of rice straw rather than the cellulose compo­nent. A 60-day pretreatment of rice straw with P. ostreatus led to a total weight loss of 25% and 41% lignin degrada­tion, but only a 17% loss of cellulose and a 48% loss of hemicellulose. A 48-h enzymatic hydrolysis lead to 52% holocellulose and 44% cellulose solubilization in the pretreated rice straw corresponding to a net sugar yield of 33% from holocellulose and 32% from cellulose.

PADDY STRAW

A recent report of a study on the pretreatment of paddy straw with the white-rot fungus T. hirsuta (Micro­bial Type Culture Collection) MTCC 136 showed high ligninase and low cellulase activities. It showed that within 10 days of solid state fermentation, the carbohy­drate content was enhanced by 11.1% and a much higher yield of sugars was obtained after enzymatic hydrolysis. Saccharification efficiency of the biologically pretreated paddy straw with the commercial enzyme Acceler — ase®1500 reached 52.69% within 72 h suggesting the delignification potential of T. hirsuta for pretreatment of lignocellulosic substrate and facilitating efficient enzymatic digestibility of cellulose (Saritha et al., 2012b).

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

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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.

Ciaran John Forde, Marie Meaney, John Bosco Carrigan, Clive Mills,

Susan Boland, Alan Hernon*

AER BIO, National Institute for Bioprocessing Research & Training (NIBRT), Blackrock, Co. Dublin, Ireland

Corresponding author email: alan. hernon@aer-bio. com

OUTLINE

Introduction 185

Sources of Biolipids 186

Plant-Derived Biolipids 186

Edible Lipids 186

Nonedible Lipids 187

Waste Edible oil 187

Animal-Derived Biolipids 188

Microalgae and Other Oleaginous Microorganisms — Derived Biolipids 189

Supply and Projected/Purrent Volume 190

Energy Balance 192

Processing of Biolipids and Properties of

Biolipid-Derived Biofuels 193

Extraction 193

Steam Distillation 193

Maceration (Solvent Extraction) 193

Enzymatic Hydrolytic Maceration 193

Expression (Cold Pressing) 194

INTRODUCTION

Biolipids have been an important source of energy since prehistoric times. While the term "biofuel" is now often synonymously used with "biodiesel", the first biofuels used were wood or other plant materials, which were burnt to provide heat, light, protection from
predators and for cooking. The earliest lamps recorded were made using plant material that was soaked with animal fat, such as lard. Later lamps, which used oils, were introduced in the eighteenth century, with early lamp fuels being oils from fish, whale and a variety of nut and other plant sources. Whale oil was much sought after for a lamp fuel as it produced a cleaner flame with

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less odor and smoke. Another source of light was candles, which were made from tallow and other oils rendered from animal waste. These fuels are known as primary biofuels, fuels that are used without any signif­icant processing in contrast to secondary biofuels where significant processing is required before the raw prod­ucts can be used as fuels. As they were discovered, coal, gas and petroleum products (kerosene in partic­ular) slowly replaced tallow and other animal-based fuels. Similarly, the use of biolipids as transport fuel is not novel; in fact, in 1900 when Rudolf Diesel showcased his internal combustion engine at The Exposition Universelle in Paris it was fuelled by peanut oil (Stauffer and Byron, 2007). However, advancements in the use of petroleum as fuel at the turn of the century resulted in the selection of this abundant, cheap and efficient hydro­carbon as the fuel of choice for transport. It was not until the oil crisis of 1973 when oil became expensive and the security of supply became paramount that biolipids were investigated again; however, this interest was short lived as the supply of crude oil from the Organization of Arab Petroleum Exporting counties was restored in 1974. Now over 100 years after Diesel’s invention we are almost completely dependent on this finite, expen­sive and polluting hydrocarbon (petroleum) as a trans­port fuel. Consequently, the use of petroleum-based products has resulted in a significant number of envi­ronmental issues including global warming via the greenhouse gas (GHG) effect. Also, in an era when it is generally accepted that we have reached peak oil production and it is projected that the demand for trans­port fuel will increase globally by 39% in the next 10 years interest in the use of biolipids as fuel has reached new heights. Recent years have seen significant research, investment and advances in sustainable en­ergy technologies such as solar, wind, geothermal, tidal and hydroelectrical. It should be noted, however, that these energy sources, along with nuclear power, relate to the generation of electricity. Currently electricity only accounts for about 33% of the world energy market, whereas liquid fuels account for the remaining 67% of global energy consumption. These figures, along with the finite nature of crude oil stocks, illustrate the need to drastically increase the production of sustainable liquid fuels (Schenk et al., 2008). Alternative liquid fuel sources are continually being sought (Bereczky, 2012; Singh and Singh, 2010) and while the obvious solution is to revert to the use of vegetable oil used in 1900, there are several problems with that approach. Most notably is the need to use arable land to feed the world’s exponen­tially growing population. Land use for the production of liquid biofuels has become a hotly debated topic since 2007 when a combination of poor harvests and alloca­tion of vast quantities of land for the production of biofuel (mostly corn ethanol) resulted in a spike in world food prices (Tenenbaum, 2008). The ease in supply of food to the world market in 2007/2008 acted at an indi­cator to what will happen in the future as the world’s population increases beyond 8 billion people and we struggle to meet the nutritional needs of humankind. It will simply be impossible to grow enough terrestrial crops to meet the worlds nutritional and energy needs. It is therefore necessary to explore the use of biolipids from all sources including lipids from plant, animal and microalgae sources. Recovering lipids from waste products like recovered vegetable oil and beef tallow will also have a role to play in meeting our insatiable demand for energy. Therefore, it is important to judicially select biolipids that require the minimum land usage (maximizing ton of oil per hectare) and lipids with good fuel properties, as discussed below. In addi­tion, the energy consumed in growing and recovering the biolipid is also an important consideration when selecting a biomass for the production of biofuel.

Biorefineries can produce chemicals and feels from biomass on several integrated platforms (Figure 14.8) (WEF, 2010).

1. The biochemical (sugar) platform, based on the biochemical conversion of biomass, focusing on sugar fermentation, and including steps dedicated to products separation and purification.

2. The thermochemical platform, based on the thermochemical conversion of biomass focusing on the gasification of carbonaceous materials and lignocellulosic biomass.

3. The microorganism platform, focusing on algae biomass cultivated in raceway type ponds or in photobioreactors.

The biorefinery concept considered by the National Renewable Energy Laboratory is based on two different primary platforms integrating various routes included in the biorefinery structure (NREL, 2009):

• The biochemical (sugar) platform performs the biomass breakdown into sugars based on chemical and biological processes:

• If lignin is the result of pretreatment and

enzymatic hydrolysis, two steps can be involved in its further transformation:

— lignin upgrading, to etherified gasoline;

— lignin pulping to high quality paper.

• If aqueous sugars result after pretreatment and enzymatic hydrolysis, they are involved in fermentation processes, resulting in ethanol, butanol, and hydrogen.

• The thermochemical platform is based on the biomass conversion onto synthesis gas through gasification, pyrolysis or hydrothermal conversion.

• Gasification results in syngas, which can be further transformed in alkanes, methanol or hydrogen by Fischer—Tropsch, catalysis, water—gas shift processes.

• Pyrolysis and hydrothermal conversion result in biooil, which is further transformed during the following processes:

— upgrading, when liquid fuel results;

— catalytic reforming, resulting in hydrogen;

— extraction, when various chemicals are obtained;

— cross-linking resulting in various (bio) materials.

The third platform—microorganism platform—has been included in the biorefineries structure by the National Renewable Energy Laboratory (WEF, 2010). This structure demonstrates that various processes can occur in a complex biorefinery, similar to a conventional oil refinery. This similarity was also graphically demon­strated by Kamm et al. (2006) (Figure 1.3).

There are also some unclassified biorefineries, which include (de Jong and van Ree, 2006) side and waste streams, MBR, most generation III biorefineries, and consortia of different industries. They are expected to play a significant role in the future, since the classic concept of biorefinery is tightly linked with the progress of agriculture, the efficiency and availability of food and feed production, with major consequences for the prime arable land (PP, 2012). Considering these problems, it is essential to promote integrated biorefinery models, which would be able to surpass the challenges address­ing retaining and recycling of phosphorous, finding new sources of soil organic carbon, maintaining biodiversity by adequate measures (PP, 2012; Star-COLIBRI, 2011). Besides, a new and challenging development began to be focused on the integrated valorization of organic waste streams, such as agrofood by-products, effluents, resulting in new value-added chemicals, biofuels, biomaterial, and water (PP, 2012; Liu et al., 2010; Visvanathan, 2010; Laufenberg et al., 2003).

This way, the integration of biorefinery platforms would be able to generate the synergism, as the underlying concept of industrial ecology. By closing material cycles and cascade utilization and recycling, it would be ensured a multilevel, explicitly integra­tive, multifunctional incorporation of raw materials, processes, and products, belonging to various industrial systems, simultaneously with preventing resource loss by source reduction and waste minimization along the entire biorefinery value chain. A full overview of the platforms, products, feedstocks and conversion pro­cesses is given in Figure 14.9 (de Jong and Marcotullio,

2010) .

Moreover, the eco-efficiency would become the lead­ing concept governing the full system, since processes for biomass treatment and conversion should be resource efficient in terms of materials and energy use and long lifetime of goods and products, along with con­sumption of auxiliaries, and should avoid adverse

Conversion Processes, Methods and Techniques Employed by Biorefineries to Transform the Raw Biomass into Commercial Products—cont’d

Optimally and flexible use of raw

materials in primary refinery

Valorization and processing of all biomass components, in

integrated and linked systems

FIGURE 14.7 Levels of integration and multifunctionality already realized in biorefineries. Source: Adapted upon Wagemann (2012). Adapted with the permission of the coordinator of "Biorefineries Roadmap as part of the German Federal Government action plans for the material and energetic utilization of renewable raw materials" brochure on behalf of The Federal Government, Professor Kurt Wagemann. (For color version of this figure, the reader is referred to the online version of this book.)

and by-products are quite numerous and diverse; a sim­ple approach for the estimation of production economics would be always opportune, so as to offer valuable in­formation about the relative feasibility of various pro­duction alternatives and routes. For example, Melin and Hurne (2011) developed an algorithm to find "the
production route with the minimum production costs for a biofuel or a chemical, for each raw material, when the process and the economic parameters occur in a known range". Other several studies have estimated biofuel production costs from corn stove through gasifi­cation and Fischer—Tropsch routes (Demirbas, 2010;

Batsi et al., 2012; Swanson et al., 2010). The objective was to compare capital investment costs and production costs for various biorefinery scenarios.

Building a bio-based economy must be able not only to solve the current economic difficulties but also to generate an economic system with minimal impact to the environment.

Even though regarded as similar to petroleum refin­ery, a comparison of the biorefinery and petrochemical value chains show some similarities but also a large number of differences. Both result in complex product trees, but one of the most relevant differences consists in compositions of fossil raw materials and biogenic raw materials (Kamm et al., 2006; Wagemann, 2012). Table 14.5 illustrates some similarities and differences between two value chains.

Consequently, for decision-making process, it is necessary to develop a methodology to drive decisions on biorefinery, with a focus on product design and pro­cess. Transition to a biorefinery economy could involve significant investments in infrastructure to produce, store and sell biorefinery products to customers (Demirbas, 2010). A number of questions related to bio­refinery diagnosis can be addressed using SWOT
analysis. Such an investigation of the opportunities and strengths, weaknesses and threats of biorefineries as developed by IEA within the Task 42 is illustrated in Table 14.6 (de Jong et al., 2009).

The concept of eco-efficiency—defined as "creating more value with less impact"—has been developed by The World Business Council to weigh and compare products and technologies in both aspects: environ­mental pressure and economic significance (WBCSD, 2000). The Organization for Economic Co-operation and Development (OECD) has defined eco-efficiency as the effectiveness with which ecological resources are used to meet human needs.

Integrating the issues concerning the environmental impacts and economic value resulting from biorefinery processes allows decision makers in the business world to evaluate and compare products and technologies simultaneously, from both points of view. Organiza­tions could be supported to establish measurable objec­tives of eco-efficiency and to facilitate comparisons between companies and business sectors by the stan­dardization of definitions and decision system for calculating and reporting eco-efficiency indicators. The environmental impact ratio, defined in Figure 14.11,

reflects how much environmental impact per environ­mental credit occurs in the product system (Hong Chua and Replace with Steinmtiller, 2010; Kim and Dale, 2004). A scenario with a greater eco-efficiency would be more sustainable, which means that it would offer more economic value per unit of environmental impact (Fig. 14.11).

Some eco-efficiency indicators were developed for different levels of biorefinery integration, following the physical flows of materials and energy (Hong Chua and Steinmtiller, 2010):

Level 1 addresses process integration and involves the key processes (receiving and preparation of feedstock, retreatment, conversion to bioproduct, and wastewater treatment system).

Level 2 refers to agriculture integration, which means that feedstocks, including agricultural waste, are supplied in the biorefinery system at the business level that is involving low costs, while biofuels, bioelectricity and biochemicals from biorefinery are sent to the agricultural sector.

Level 3 involves livestock farming integration at the business level, meaning that the organic waste from farms are supplied to the biorefinery system, while animal feed products are sent to the farm.

Estimated costs of production in biorefinery sys­tems may be hampered by a number of driving forces who can change their direction of action and/or importance in time (agricultural development, raw material costs, production scale, competing markets evolution, their demands and access, waste recovery

TABLE 14.6 SWOT[1] Analysis of Biorefineries Processes (de Jong et al., 2009)

Weaknesses

• Adds value to the sustainable use of biomass

• Maximizes biomass conversion efficiency—minimizing raw material requirements

• Produces a spectrum of bio-based products (food, feed, materials, and chemicals) and bioenergy (fuels, power and/or heat) feeding the full bio-based economy

• Strong knowledge of infrastructure available to tackle any nontechnical and technical issues potentially hindering the deployment trajectory

• Is not new, and in some market sectors (food, paper, etc.), it is common practice

Opportunities

• Make a significant contribution to sustainable development

• Challenging national, European and global policy goals—international focus on sustainable use of biomass for the production of bioenergy

• Biomass availability is limited so the raw material should be used as efficiently as possible—i. e. development of multipurpose biorefineries in a framework of scarce raw materials and energy

• International development of a portfolio of biorefinery concepts, including designing technical processes

• Strengthening of the economic position of various market sectors (e. g. agriculture, forestry, chemical and energy)

• Broad undefined and unclassified area

• Needs involvement of stakeholders from different market sectors (agro, energy, chemical,.) over the full biomass value chain

• Most promising biorefinery processes/concepts not clear

• Most promising biomass value chains, including current/future market volumes/prices, not clear

• Still at a stage of studying and concept development instead of real market implementation

• Variability of quality and energy density of biomass

Threats

• Biorefinery is seen as hype that still has to prove its benefits in the real market

• Economic change and drop in fossil fuel prices

• Fast implementation of other renewable energy technologies filling market needs

• No level playing field concerning bio-based products and bioenergy (assessed to a higher standard)

• Global, national and regional availability and contractibility of raw materials (e. g. climate change, policies, and logistics)

• High-investment capital for pilot and demonstration initiatives difficult to find, and existing industrial infrastructure is not depreciated yet

• Fluctuating (long-term) governmental policies

• Questioning of food/feed/fuels (land use competition) and sustainability of biomass production

• Goals of end users often focused upon single product

and recycling alternatives, storage and production costs, distribution costs, etc.), which could be associ­ated with the components of a complex system with various boundaries (Figure 14.12; Demirbas, 2010; Kim and Dale, 2004).

Life cycle assessment (LCA) is an especially useful tool to investigate the environmental performance of

product and/or technologies. The problem to be solved in the case of biorefineries is not a simple one because these systems are characterized by some particularities that need to be considered in evaluating the processes on an LCA basis and to ensure correct results in terms of eco-efficiency (for example, sometimes it is not obvious which product should be the main output;

FIGURE 14.11 Integration of economic analysis and environmental impact for eco-efficiency (ADP, abiotic resources depletion potential; GWP, global warming potential; ODP, ozone layer depletion po­tential; POCP, photochemical oxidation potential; HTP, human toxicity potential; ETP, ecological toxicity potential; AP, acidification potential; EP, eutrophica­tion potential; NPV, net present value; IRR, internal rate of return). (For color version of this figure, the reader is referred to the online version of this book.)

Environmental impact Environmental credit

Hong Chua and SteinmUller, 2010; Laser et al., 2009). Further, the system boundaries could be different if the biorefineries are nonintegrated or integrated and this can determine the selection of system boundaries, which could also affect the eco-efficiency results, while allocation issues in particular are both important and somewhat controversial (Figure 14.12). A very common approach considers that all biomass is local since this could improve the selection of crops and cropping systems for local biorefineries, reduce opportunities for agenda — driven manipulation of data and opportunities for system integration and waste utilization could be better exploited (Kim and Dale, 2004). The functional unit could be chosen as unit area of land allocated for crop biomass for a certain time period since cropping systems play an important role in the environmental performance of bio-based products, while impacts assessment could address global warming potential, nonrenewable energy, crude oil con­sumption, water use, acidification, eutrophication, biode­gradability, less toxicity, etc. (Laser et al., 2009; Demirbas,

2010) . In their study, Hong Chua and Steinmuller (2010) have identified the following main environmental influences for a biorefinery: energy consumption, material consumption, GHG emissions, acidification, and eutro­phication. The eco-efficiency indicators used to account for these environmental influences are as shown in Table 14.7.

Ensuring biorefinery eco-efficiency is one of the most relevant objectives of Task 42 of IEA in parallel with the projection of new perspectives in terms of competitive­ness, sustainability, and safety of processing routes for biogenic raw materials to guarantee the concurrent fabrication of biofuels, commodity chemicals, new mate­rials, heat and power.

Room temperature ionic liquids (RTILs) were used for the development of new technologies in chemical and biological transformations, separations, and more recently biomass pretreatment. RTILs consist of an organic cation and an organic or inorganic anion. This tremendous variation allows solvent properties to be tailored to specific applications such as biocatalysis, particularly as nonaqueous alternatives to organic solvents. More recently, RTILs have been used as alter­natives for lignocellulosic pretreatment (Mora-Pale et al., 2011). Birch wood was pretreated with N-methylmorpholine-N-oxide (NMMO or NMO) fol­lowed by enzymatic hydrolysis and fermentation to ethanol or digestion to biogas. The pretreatments were carried out with NMMO at 130 °C for 3 h, and the effects of drying after the pretreatment were inves­tigated (Goshadrou et al., 2013). Another interesting process is the use of concentrated phosphoric acid (CPA) in the pretreatment of lignocellulosic biomass (Zhao et al., 2012). After reprecipitation from CPA cellu­lose becomes completely amorphous and contains little lignin and hemicellulose. Further research is needed to evaluate and improve the economics of usage of ionic liquids (ILs), NMMO and CPA for pretreatment of lignocellulosic biomass. Also the integration with subsequent chemocatalytic and enzymatic/fermenta — tive processes such as simultaneous saccharification and fermentation needs further research. Especially, the ability of microorganisms to ferment sugars in the presence of these solvents also needs to be tested to carry out a continuous process. ILs are still very expen­sive and need to be synthesized at a much lower cost and on a much larger scale. Other points of concern are the buildup of inorganics in the ILs introduced with the lignocellulosic biomass (especially a concern with nonwoody lignocellulosic biomass such as straw and bagasse) and chemical modifications of the ILs. So it is rather questionable if the great potential assigned to ILs can be fulfilled for bulk applications such as biomass pretreatment taking into account the aforementioned limitations.

Lignocellulosic biomass pretreatment in RTIL’s is an alternative showing promise, with comparable or supe­rior yields of fermentable sugars, than conventional pre­treatments. The high number of RTILs that can be synthesized allows the design of solvents with specific physicochemical properties that play a critical role inter­acting with lignocellulosic biomass subcomponents. Today, these interaction mechanisms are better under­stood. However, future challenges rely on the ability to make this process economically feasible. This might be achieved by optimizing large-scale pretreatment condi­tions, performing post-pretreatment steps in RTILs, reusing RTILs, recycling the RTILs with reduced energy consumption and enhancing process efficiency, and pro­ducing high-value biobased products and chemicals in addition to ethanol. Moreover, the potential high value of lignin suggests that it might instead be used in the large-scale diversified manufacture of high-value chem­icals, traditionally obtained from petroleum (Mora-Pale et al., 2011).

Aspartame is a methyl ester of dipeptides consisting of aspartic acid and phenylalanine. It was accidentally discovered in 1965 by the chemist James M. Schlatter while working on an antiulcer drug (Walters, 1991). Aspartame is 160—220 times sweeter than sucrose and is used as arti­ficial sweetener in foods and beverages. Aspartame is pro­duced by coupling microbial fermentation and synthesis. Phenylalanine and aspartic acid are produced by microbi­al fermentation and phenylalanine is reacted with meth­anol to form the methyl ester. Aspartic acid is also treated in such a way as to protect active sites by benzyl rings. Then the modified amino acids are mixed in a reac­tion tank at appropriate temperatures to get aspartame in­termediates (Figure 19.2). It is further treated with acetic acid, purified, crystallized and powdered to produce aspartame. Methods for direct enzymatic synthesis and chemical synthesis are also reported.

One of the most remarkable events in investigating H2 metabolism in green algae was the discovery of sus­tained H2 photoproduction in C. reinhardtii cultures un­der sulfur-deprived conditions (Melis et al., 2000; Ghirardi et al., 2000). In this approach, the long-term H2 photoproduction is possible due to a metabolic switch occurring in sulfur-deprived algal cells, which separate temporarily the O2-evolving, aerobic (Eqn (21.5)) and H2-producing, anaerobic (Eqn (21.6)) stages in the same culture.

Sulfur deprivation causes the partial and reversible inhibition of PSII-dependent water-splitting activity in algae. As demonstrated by Wykoff et al. (1998), C. rein — hardtii cells lose gradually up to 75% of the initial PSII ac­tivity within the first 24 h of sulfur starvation. The reduction of H2O-splitting activity was also shown under deprivation of other nutrients, such as nitrogen, phosphorus, Fe and Mn (Wykoff et al., 1998; Ghirardi et al., 2000; Philipps et al., 2012) but usually with a significant delay, as compared to sulfur starvation. The repression of the linear electron flow from the PSII centers under nutrient starvation is a common phenom­enon not only for green algae but also for cyanobacteria (Sauer et al., 2001) and high plants (Dietz and Heilos, 1990; Ferreira and Teixeira, 1992), and is a good example of how photosynthetic organisms adjust the rate of photosynthesis to the stress conditions. Continuous nutrient starvation reduces the capacity for de novo pro­tein biosynthesis and CO2 fixation, and, as a result, decreases the demand of the cells in the photosynthetic reductants (Grossman, 2000). Under these conditions, the repression of the O2-evolving activity and linear electron flow protects the photosynthetic apparatus from overreduction, generation of reactive oxygen spe­cies and photoinhibition. Numerous experiments showed that the inhibition of O2-evolving activity in nutrient-deprived cells is mostly caused by the loss of PSII centers (Kolber et al., 1988; Wykoff et al., 1998). In the absence of basic nutrients such as nitrogen, phos­phorus or sulfur, the cells cannot efficiently resynthesize D1 protein, the key component of the PSII complex, and the PSII repair cycle is blocked (Melis and Chen, 2005).

Nutrient deprivation, however, has little effect on cellular respiration, especially in the first few days (Melis et al., 2000). As a result, the rate of photosynthetic

O2 evolution falls below the rate of respiratory O2 up­take and algal cultures, if sealed in photobioreactors with a little headspace volume, become anaerobic in the light (Melis et al., 2000). In sulfur-deprived cultures, this usually happens within the first 24 h. The establish­ment of anaerobiosis in the sealed photobioreactor induces the expression of [Fe—Fe]-hydrogenase

enzymes in algal cells (Happe and Kaminski, 2002; Forestier et al., 2003). [Fe-Fe]-hydrogenase accepts elec­trons from the photosynthetic electron-transport chain and algae start producing H2 in the light. If not opti­mized, H2 photoproduction lasts for several days (Melis et al., 2000; Ghirardi et al., 2000). Under continuous flow of the medium containing sulfur in a micromolar range, algae produce H2 gas for several months, although at substantially low rates (Fedorov et al., 2005; Laurinavi — chene et al., 2006). The most interesting results obtained from sulfur-deprivation experiments are summarized in Table 21.1. As shown in the table, the rates and the yields of H2 photoproduction in algal cultures vary depending on the experimental conditions. In C. reinhardtii wild — type strains the rate usually does not exceed 13 mmol mg/Chlh, while some genetically modified strains are able to produce H2 with rates up to 27 mmol mg/Chl h.

In green algae, sulfur deprivation demonstrates the strongest inhibitory effect on PSII (Wykoff et al., 1998; Ghirardi et al., 2000) most probably due to the lowest intracellular sulfur reserves. The later studies showed that the same principle works for phosphorus — deprived (Batyrova et al., 2012) and nitrogen-deprived (Philipps et al., 2012) microalgae. Phosphorus-depleted cultures start producing H2 gas only after the initial growth period on the phosphorus-free medium. Growing algae utilize an intracellular pool of reserved phosphorus. When they reach the point of phosphorus starvation, PSII in algal cells is inactivated in a manner similar to sulfur-starved algae. Despite a considerable delay in the establishment of anaerobic conditions, phosphorus-deprived algae produce only slightly less H2 gas than sulfur-deprived cultures under the same experimental conditions, but they also accumulate less starch reserves during the growth stage (Batyrova et al., 2012). Nitrogen-deprived algae behave in a similar way. They produce H2, but with a significant delay (Philipps et al., 2012). In contrast to phosphorus — deprived cells, the delay in nitrogen-deprived cultures seems to be caused by slower inactivation of PSII cen­ters. These algae also accumulate significantly more starch reserves than sulfur-deprived algae, but degrade them slower. As a result, they produce considerably less H2 overall. Inability to efficiently channel electrons from carbohydrate oxidation toward the hydrogenase enzyme likely causes the degradation of the Cyt bg/ complex upon nitrogen starvation and lowers amounts of PetF. Nevertheless, nitrogen-deprived cultures may have a higher potential for the light-independent H2 production pathway (Philipps et al., 2012).

The vast majority of experiments completed on H2 production by nutrient-deprived microalgae have been undertaken so far with C. reinhardtii cultures. However, other species of green algae also produce H2 gas under this condition (Winkler et al., 2002; Skjanes et al., 2008; Meuser et al., 2009). Successful H2 production has been demonstrated by sulfur-depriving C. noctigama and Chlamydomonas euryale (Skjanes et al., 2008). Sulfur — deprived cultures of S. obliquus, Platymonas subcordi/or — mis, Scenedesmus vacuolatus, Chlamydomonas vectensis, Chlamydomonas pyrenoidosa, Desmodesmus subspicatus, Pseudokirchneriella subcapitata, Chlamydomonas moewusii and Lobochlamys culleus generate only minor amounts of H2 gas (Winkler et al., 2002; Guan et al., 2004; Skjanes et al., 2008; Meuser et al., 2009). Some other tested spe­cies, such as Dunaliella salina and C. vulgaris demonstrate no detectible hydrogenase activities and do not produce H2 under sulfur-deprived conditions (Cao et al., 2001; Winkler et al., 2002).

H2 photoproduction in nutrient-deprived algae de­pends both on the residual PSII activity remaining in cells after inactivation (Antal et al., 2003; Kosourov et al., 2003) and on the catabolism of starch accumulated during the first 18—24 h of sulfur deprivation (Fouchard et al., 2005; Ghirardi et al., 2000; Kosourov et al., 2003; Tsygankov et al., 2002; Zhang et al., 2002). The contribu­tion of these two pathways in H2 photoproduction varies depending on the stage of sulfur deprivation (Laurinavichene et al., 2004) and, most probably, on the strain used in the experiment (Chochois et al., 2009). In the wild-type C. reinhardtii CC-124 strain, starch degradation may donate up to 20% electrons to hydrog- enase enzymes in the middle of the H2 production stage (Kosourov et al., 2003). Besides contribution to H2 photoproduction, the degradation of starch and other stored organic substrates fuels the respiratory consump­tion of O2 produced by the residual PSII activity and therefore is responsible for maintaining culture anaero — biosis and for protecting hydrogenase enzymes from O2 inactivation (Fouchard et al., 2005; Kosourov et al., 2007). The importance of efficient respiration for H2 photoproduction was further proved by inhibitory analysis (Antal et al., 2009) and in the respiratory — deficient mutants (Table 21.1).

Under photoheterotrophic conditions (when acetate is the only substrate), accumulation of starch in algae in the beginning of sulfur deprivation is tightly linked to consumption of acetate from the medium. The respi­ration of acetate provides the cells with a substrate for CO2 fixation. It also helps with the establishment of anaerobiosis in the photobioreactor (Kosourov et al., 2007). The use of acetate in the growth medium, however, increases the expense associated with maintenance of

TABLE 21.1 The Rates and Yields of H2 Photoproduction by the Sulfur-Deprived, Wild-Type C. reinhardtii Strains and Some Mutants under Different Experimental Conditions

the system and should therefore be avoided. Recently, Tsygankov et al. (2006) showed that H2 photoproduc­tion in green algae is also possible under autotrophic conditions, when cultures are supplied with CO2 gas instead of acetate. In this experiment, authors used the microprocessor-controlled bioreactor system for a controllable addition of CO2 gas. The unique aspect of this system is that cells are provided with appropriate amounts of CO2, in accordance with the demands of the culture. Under these conditions, algae accumulate enough starch that can later be used for the establish­ment of anaerobiosis in the culture and for the removal of O2 during the H2 production stage (Kosourov et al., 2007). Using the special light regime, the authors gener­ated almost the same amounts of H2 gas as in photoheter­otrophic cultures (Tsygankov et al., 2006; Tolstygina et al., 2009).

LuizJ. Visioli, Fabiane M. Stringhini, Paulo R. S. Salbego, Daniel P. Chielle,
Gabrielly V. Ribeiro, Juliana M. Gasparotto, Bruno C. Aita, Rodrigo Klaic,

Jessica M. Moscon, Marcio A. Mazutti*

Department of Chemical Engineering, Federal University of Santa Maria, Santa Maria, Brazil
* Corresponding author email: mazutti@ufsm. br

OUTLINE

INTRODUCTION

The last years have verified a pronounced demand for fossil fuels worldwide due to increase in industriali­zation and motorization (Agrawal et al., 2007). Nowa­days, fossil fuels represent around 80% of all primary energy consumed in the world, where 58% is employed in the transport sector (Escobar et al., 2009). The esti­mates show that the global energy demand is projected to grow by more than 50% by 2025, with much of this in­crease in demand emerging from several rapidly devel­oping nations. Clearly, increasing demand for finite petroleum resources cannot be a satisfactory policy for the long term (Ragauskas et al., 2006).

Biofuels are a renewable energy source produced from natural (plant) materials, which can be used as a substitute for petroleum fuels (Demirbas, 2011). The global demand for liquid biofuels more than tripled in last decade, indisputably showing the increasing trend
toward the use of fuels derived from plant feedstock (Ferreira-Leitao et al., 2010).

Agroindustrial and forestry residues, which are by­products of key industrial and economical activities, stand out as potential raw materials for the production of renewable fuels, chemicals and energy (Ferreira- Leitao et al., 2010). Biofuels can also be derived from fishery products or municipal wastes, also including by-products and wastes originated from agroindustry, food industry and food services (Nigam and Singh,

2011) . The key advantage of the utilization of renewable sources for the production of biofuels is the utilization of natural bioresources (that are geographically more evenly distributed than fossil fuels) and the produced bioenergy provides independence and security of en­ergy supply (Nigam and Singh, 2011). The use of agricul­tural residue and waste substrates as raw materials is advantageous as their availability is not hindered by a requirement for arable land for the production of food

Bioenergy Research: Advances and Applications http://dx. doi. org/10.1016/B978-0-444-59561-4.00003-6

and feed. Reusing agricultural waste products is one goal of environmental sustainability and has become an option to add value to producers (Manique et al.,

2012) . In addition, waste utilization prevents its accumu­lation, which is of great environmental concern due to its potential for contamination of rivers and underground water (Ferreira-Leitao et al., 2010).

The most well-known first-generation biofuel is ethanol (Nigam and Singh, 2011), which is currently be­ing produced from sugarcane or corn and will often be referred to as bioethanol (Demirbas, 2011). Ethanol has long been considered as a suitable alternative to fossil fuels either as a sole fuel in cars with dedicated engines or as an additive in fuel blends with no engine modifica­tion requirement when mixed up to 30%. Today, bio­ethanol is the most dominant biofuel and its global production showed an upward trend over the last 25 years. Worldwide production capacity in 2006 was about 49 x 109 liters per year, and total output in 2015 is forecast to reach over 115 x 109 liters (Talebnia et al., 2010).

Feedstock containing significant amounts of sugar, or materials that can be converted into sugars, such as starch or cellulose, can also be used in the production of ethanol (Nigam and Singh, 2011). The production of ethanol from cellulosic feedstock has a growing interest worldwide. Cellulosic biomass is an abundant renew­able resource on earth and includes various agricultural residues. Some of these agricultural residues such as straw, corn husk, and sugarcane residue represent an abundant, inexpensive, and readily available source of renewable lignocellulosic biomass. At the present time, this readily available biomass is considered as a waste and is disposed of through agricultural burning after harvest (Dawson and Boopathy, 2007).

Agricultural residues are produced in large quantities throughout the world. Approximately, 1 kg of residue is produced for each kilogram of grain harvested. These residues are renewable resources that could be used to produce ethanol and many other value-added products (Dawson and Boopathy, 2007). Among these residues, ethanol can be produced from biomass feedstocks such as sucrose-containing feedstocks (e. g. sugar beet, sweet sorghum, and sugarcane), starchy materials (e. g. wheat, corn, barley, cassava, and rice), and lignocellulosic biomass (e. g. wood, straw, grasses, and various crop res­idues). These biomass feedstocks can reduce about 50% of the price of the ethanol produced, depending on the type of the biomass used (Hong and Yoon, 2011).

Lignocellulosic waste materials obtained from energy crops, wood and agricultural residues represent the most abundant global source of renewable biomass. Among the agricultural residues, wheat straw is the largest biomass feedstock in Europe and the second largest in the world after rice straw. About 21% of the world’s food depends on the wheat crop and its global production needs to be increased to satisfy the growing demand of human consumption; therefore, wheat straw would serve as a great potential feedstock for produc­tion of ethanol in the twenty-first century (Talebnia et al., 2010).

The use of lignocellulosic energy crops, and particularly low-cost biomass residues, offers excellent perspectives for large-scale application of ethanol in transportation fuels. These materials will increase the ethanol production capacity and reduce production cost to a competitive level. Bioethanol from these materials provides a highly cost — effective option for CO2 emission reduction in the trans­portation sector (Patle and Lal, 2007).

The utilization of lignocellulosic biomass has been closely associated with a new technological concept, so-called biorefinery, which emerges as key to the significant expansion of the desired production of ethanol. Fermentative processes stand out, where microbial metabolism is used for the transformation of simple raw materials in products with high aggre­gate value. Experts believe that the biorefineries are likely to be a key industry of the twenty-first century, even responsible for a new industrial revolution, because of the importance of the technologies they employ and their effects on the actual industrial model (Santos et al., 2010).

Regarding crop residues that have proper application in energy supply, the energetic generation cost for useful energy is a matter for consideration. Studies done so far suggest that, when transport distances are similar, the most efficient energetic use of lignocellulosic materials such as agricultural residues is the application for the generation of electricity. Applied in this way, crop resi­dues are most efficient in replacing fossil fuels, much more so than when crop residues are converted to ethanol for use in cars. However, when road transport distances to power-generating plants are very large, it may be that energetic uses that require a much lower input of transport fuel become energetically more attrac­tive (Reijnders, 2008).

Based on these aspects, the main objective of this work is to present an overview about bioethanol production from agroindustrial residues, which were based on low-priced feedstocks such as crop residues, municipal/industrial solid waste, and food residues. For this purpose, papers and overviews since 2006 were reported.

Yanbin Yin

Department of Biological Sciences, Northern Illinois University, DeKalb, IL, USA

email: yyin@niu. edu

The major components of plant biomass are carbohydrate-rich cell walls, composed of different bio­polymers such as polysaccharides and lignins as well as some minor wall structural glycoproteins (Somerville et al., 2004). Biomass used for biofuel production is pri­marily derived from secondary cell walls. For example, wood cells from poplar trees contain a thin layer of pri­mary cell walls and multiple layers of much thicker and tougher secondary cell walls. All plant cells of different tissues have primary cell walls while only in developed cells (stopped growing) secondary cell walls appear (Cosgrove, 2005). The chemical compositions in primary and secondary cell walls differ significantly (Mohnen et al., 2008). The primary cell wall contains no lignins and the polysaccharides include celluloses, hemicellu — loses (primarily xyloglucans and mannans in dicots and xylans in monocots) and pectins. However, in the secondary cell walls, there are higher percentage of cellu­loses, different hemicellulosic polysaccharides (primar­ily xylans in both dicots and monocots) and lignins. For example, wood secondary cell walls contain 35—50% celluloses, 25—30% hemicelluloses (mostly xylans) and 15—30% lignins (Himmel et al., 2007).

pectins both refer to a collection of complex polysaccha­rides mostly with side chains. Hemicelluloses contain four major groups: xyloglucans, mannans, xylans and mixed-linkage glucans, while pectins contain three ma­jor groups: galacturonans, rhamnogalacturonan I and rhamnogalaturonan II. Each of the groups of hemicellu — loses and pectins do not refer to a single type of polysac­charides; they often refer to polysaccharides with the same backbone structure (sugars and linkages) while with different side chains. Due to this reason, all these biopolymers are cross-linked and interwoven (Somerville et al., 2004; Himmel et al., 2007) to form very complex and heterogeneous cell wall structures. Particularly celluloses are wrapped by hemicelluloses and buried in a lignin network and not accessible to enzymes so that the degradation efficiency is very low if no costly chemical pretreatment is applied.

Although celluloses are simple polymer of glucose linked by beta-1,4,-glucosidic bond, the complexity of chemical compositions of hemicelluloses and pectins is remarkably high (Somerville et al., 2004). The reasons are as follows: (1) there are 14 different monosacchar — aides (sugar units) found in hemicelluloses and pectins (Pauly and Keegstra, 2008b); (2) the possible glycosidic

Bioenergy Research: Advances and Applications http://dx. doi. org/10.1016/B978-0-444-59561-4.00006-1

linkages formed between two sugars are extremely diverse as theoretically they can be connected between any hydroxyl group of two sugars and (3) sugars in the polysaccharides can be further modified by, e. g. methylation, acetylation or esterification.

Lignins, however, are complex heterogeneous phenolic polymers and chemically very distinct from polysaccharides. They are formed by three major monomers: hydroxyphenyl (H), guaiacyl (G) and syringyl (S) units, which are derived from coumaryl alcohol, coniferyl alcohol and sinapyl alcohol, res­pectively (Boerjan et al., 2003). All the biopolymers in plant cell walls are cross-linked and interwoven (Somerville et al., 2004; Himmel et al., 2007) to form very complex and heterogeneous structures, which is believed to enable cell walls recalcitrant to enzymatic degradation.

Besides, cell wall compositions and structures also vary from tissue to tissue. The reason is because the cell wall biosynthetic enzymes are differentially regu­lated and expressed in different tissues. Furthermore, different plants, especially those of distant evolutionary clades, have very distinct cell wall biopolymer composi­tions. For example, grasses generally have significantly higher percentage of xylans than trees (Pauly and Keegstra, 2008a).

For biofuel production, polysaccharides especially celluloses are favored as their degradation releases fermentable sugars. Lignins, however, are phenolic polymers and chemically distinct from polysaccharides, not giving rise to sugars and meant to be removed in the biofuel production. In order to develop transgenic plants with modified cell wall compositions (i. e. higher cellulose and lower lignin content), we need a better un­derstanding of how plant cell wall polysaccharides and lignins are synthesized.

Lignocellulosic biomass includes corn stover, straw, wheat stover, algae and others. The primary components in lignocellulosic biomass are cellulose, hemicellulose and lignin. Compositions differ for different types of biomass. Lignocellulosic biomass is considered unfer­mentable because most microbes cannot degrade it

without pretreatment and lignin is optimally degraded under aerobic conditions via several dioxygenase — type enzymes, although some anaerobic bacteria can degrade it, albeit slowly. Pretreatment methods include mechanical, hydrothermal, biological, chemical, ammo­nia or supercritical CO2 explosion and ionic liquid extraction (Gu, 2013). An MFC using corn stover after steam-explosion pretreatment as the substrate achieved a maximum power density of 861 mW m~2 (Zuo et al.,

2006) . MFCs fed with Chlorella vulgaris and Ulva lactuca powders achieved maximum power densities of 0.98 Wm-2 (277 W m~3) and 0.76 Wm-2 (215 Wm-3), respectively (Velasquez-Orta et al., 2009).

Cellulose is relative easy to utilize by MFCs compared with lignocellulosic biomass. A maximum power den­sity of 272 mW m~2 was achieved using carboxymethyl cellulose as substrate in an MFC (Rezaei et al., 2009). This means that it is possible to utilize the tissue paper (cellulose) in municipal wastewater as substrate. Table 9.3 shows the list of substrates used for MFCs studied until 2013.

SUMMARY AND PERSPECTIVES

This chapter discusses the operating principles of MFCs and various aspects in bioelectrochemistry in MFC research. Although tremendous advances have been made around 2013 in academic MFC research including a much better understanding of biofilm electro­chemistry and better reactor designs, major technological hurdles remain for practical MFC applications beyond powering sensor devices. It is unreasonable to expect MFCs to reach power densities on par with those from chemical fuel cells because MFCs are powered by low- energy-density fuels such as dilute organic matter in wastewaters. However, it is still necessary to increase MFC power generation to what would be considered a useful level (e. g. to offset part of the energy input in wastewater treatment), much higher than what has been achieved.

Various approaches have been attempted to increase MFC performance including improved reactor designs, electrode and membrane materials, feedstock selection and modification, introduction of exogenous mediators, and utilization of secreted endogenous mediators. Unfortunately, many of the improvements come with inherent cost increases with little hope for practical applications. Some MFC researchers have come to realize that a breakthrough in biofilm engineering should be explored. Recent discoveries such as interspe­cies electron transfer, conductive cell aggregates and long-distance conductive filaments provide new hope for means to engineer robust "super-bug" biofilms with greatly enhanced electron transfer capacity and a voracious appetite for complex organic matter digestion. The dawn of a new era for MFC research might be in sight and the synergistic involvement of biochemical and environmental engineers, microbiologists and mo­lecular biologists may soon bear fruit in this exciting field of practical research.