Adrian Muller

Research Institute of Organic Agriculture FiBL, Zurich, Switzerland; Institute for Environmental Decisions,
Swiss Federal Institutes of Technology (ETH), Zurich, Switzerland
email: adrian. mueller@fibl. org

OUTLINE

Introduction 407

Criteria for Sustainable Farming and Sustainable

Food Systems 409

Conventional Agricultural

Production 409

Sustainable Agricultural Production 409

Sustainable Food Systems 410

What is Sustainable Bioenergy

Production? 410

Ways of Comparisons 410

Sustainability Criteria for Biofuel Production 410

Biomass Use 412

How Much Bioenergy may be Produced

Sustainably? 412

Global Bioenergy Potential 413

Bioenergy Potential on Farm Level 414

Conclusions 415

References 415

INTRODUCTION

Is bioenergy a sustainable energy source? A positive answer to this question is a key if bioenergy shall become a significant sustainable energy source for future societies. The answer to this question depends on various aspects of the production and use of bio­energy. The most prominent topic there is the green­house gas (GHG) balance, as this is the key motivation to investigate bioenergy at all. For most cases, the GHG balance is positive, albeit not at a tremendously high rate and negative values are due to large emissions from direct and indirect land use change (ILUC), e. g. if palm oil plantations are estab­lished on peatland rainforest (Faist Emmenegger et al., 2012; PBL, 2010; Fargione et al., 2008). Clearly, in the use phase, GHG emissions are counted as zero due to the overall assumption of renewable biomass provision for bioenergy. However, over the whole life
cycle of bioenergy, GHG emissions arise at various steps, in particular, in the agricultural production phase (Faist Emmenegger et al., 2012). The GHG bal­ance usually plays the role of a fundamental decision criterion in favor or against bioenergy types. With a zero or negative balance, bioenergy will not contribute to and may even adversely affect climate change miti­gation. However, even in this case, one could promote one argument for bioenergy, namely that it replaces nonrenewable energy sources with renewable ones. This is particularly attractive for liquid fuels as there are currently no other alternatives available than liquid fuels from biological sources. In the following, we often focus on liquid biofuels as the discussion of sustainabil­ity of bioenergy is developed furthest for those and most data are available for those bioenergy types. The findings on the sustainability of agricultural produc­tion of crops for liquid biofuel, however, apply to agri­cultural production of any bioenergy type and we will

Bioenergy Research: Advances and Applications http://dx. doi. org/10.1016/B978-0-444-59561-4.00023-1

also report findings for biogas production, for example, on which there is also a considerable amount of research.

Besides the GHG balance, many other criteria are needed to assess the sustainability of bioenergy. They range from environmental impacts of the agricultural pro­duction process over emissions in the use phase (e. g. nitrous oxide emissions from biomass-fueled power plants) to socioeconomic aspects, such as production costs or effects on labor (Elbehri et al., 2013; Faist Emmenegger et al., 2012; Delucchi, 2010). The assessments of environ­mental impacts show that most bioenergy crops perform worse than the fossil fuel baseline regarding many criteria of sustainability in agricultural production. The best per­formance is realized for some residue or forestry-based bioenergy sources such as fuels from wood products (Faist Emmenegger et al., 2012). The negative perfor­mance of bioenergy crops is due to the fact that current conventional agricultural production has many adverse environmental effects (Matson et al., 1997; IAASTD, 2009; see e. g. Tomei and Upham, 2009 on soy-based biodiesel in Argentina for an exemplification).

As important as these findings on environmental per­formance is, the key point in the current discussion besides the GHG balance is land competition between bioenergy crops and food production (Rathman et al., 2010; Delzeit et al., 2010; HLPE, 2013). Bioenergy crops compete for fertile land with food crops and bioenergy use of food crops such as maize directly competes with food use. This dynamics has been behind the vola­tile and high food prices in recent years as, e. g. for maize and grain in 2007-2008 (NYT, 2007; HLPE, 2013).

There are several options available to address all these challenges. First, there are alternative ways of agricul­tural production that reduce the adverse impacts from farming, a key example being organic agriculture (Rossi, 2012). These alternatives usually focus on systemic as­pects of the whole production systems, emphasize closed nutrient cycles, sustaining soil fertility and plant health and the role of ecosystem services, e. g. for pest and dis­ease control. Second, how strong land competition may become depends on the concrete situation, i. e. on total bioenergy demand, relative prices for food and energy, the policy environment with incentives and regulations, etc. Very circumspect planning of any larger scale bio­energy strategy seems crucial to have a chance to avoid such adverse effects (e. g. Damen, 2010 for Thailand, wa Gathui and Ngugi, 2010 for vulnerable regions in Kenya, or Palola and Walker, 2010 on oil palms). Standards and certification are also suggested as a means to address the potential land use competition, e. g. by explicitly excluding bioenergy from fertile croplands (e. g. GEF et al., 2013). Third, new forms of bioenergy emerge (IEA, 2010). Instead of the so-called "first-generation" bioenergy that is based on use of oil, starch or sugar contents of crops to manufacture biodiesel, "second-gen­eration" bioenergy relies on the lignocellulosic contents of the crops. This allows a much wider range of crops to be utilized for biofuel production, in particular nonfood woody crops such as switchgrass or Eucalyptus (IEA, 2010). It also allows utilization of basically the whole plant and of crop residues for biofuel production. This reduces land demand per unit energy for bioenergy. However, it is not expected that the second-generation biofuels will play a significant role in the near future as much research is still needed (IEA, 2010).

An aspect that is largely missing in this discussion on environmental impacts of bioenergy production and land competition is the role of biomass quantities. Large quan­tities of biomass are needed for bioenergy strategies that significantly contribute to the global energy supply. On the other hand, biomass plays a key role as a fertilizer in sustainable agricultural production systems. There is thus a direct competition for biomass between exporting it from the agricultural production system for bioenergy use and recycling it as a fertilizer (Muller, 2009). Data and studies on how much biomass may be exported for bioenergy use from sustainable agricultural farming sys­tems are scarce, but indications that it will not be much dominate, as we will discuss further down.

The key topic in this discussion is thus less whether agricultural production of bioenergy crops can be done in a sustainable way, as sustainable agricultural produc­tion is well established and can be implemented for any crop production and as options to reduce land competi­tion are around in principle, albeit challenging to imple­ment. The question is thus rather how much bioenergy can be sustainably produced in such a context where biomass is not a waste output from agricultural produc­tion but an essential fertilizer input in sustainable agri­cultural production systems and where fertile land is needed for food production.

In this chapter, we focus on the sustainable production of bioenergy crops with a focus on the farm level. This thus covers the farm operations, but it does not cover processing, transport, storage and use of bioenergy. Possible disposal of waste after bioenergy use is shortly addressed in the context of fertilizer use (cf. Section Bioenergy Potential on Farm Level). This chapter also covers more aggregate aspects of agricultural production such as land and water resource use and more systemic aspects related to the whole food system, such as the competition for land, water and biomass between food and bioenergy production. In this context, we also address general aspects of food security and in the con­clusions also relate to the role of meat and milk produc­tion as another sector that competes for scarce land resources with crop-based food production.

We restrict this analysis to agricultural bioenergy. We thus address forestry only occasionally and do not

address aquaculture such as for algae-based biomass pro­duction for energy use or biomass production in indus­trial contexts, such as in bioreactors (Chen et al., 2011). As biological processes are involved in all these cases, the key topics we discuss in the following are relevant also there: resource use and resource competition, envi­ronmental impacts of the production (e. g. water and air pollution, intoxication due to pesticide use, etc.), and ef­fects on the sustainability of the food production system as a whole. On the other hand, harvesting the energy cap­ture potential of photosynthesis in purely industrial pro­duction of biomass in bioreactors could be an option, as the production system can work well separated from the natural environment. For such production, no ecosystem inputs such as fertile soils and water bodies are used as inputs or sinks (e. g. for nutrient runoff) and thus potentially depleted. Environmental impacts can thus be kept to a minimum. Such operations do however not refer to agricultural but rather to industrial produc­tion of biomass for bioenergy use and we do not further pursue this here. Furthermore, they are still in the research and development phase and commercial use is not expected in the near future (Pragya et al., 2013).

This chapter is organized as follows. In Section Criteria for Sustainable Farming and Sustainable Food Systems, we discuss the criteria of sustainable agricul­tural production and sustainable food systems. In Sec­tion What is Sustainable Bioenergy Production? we assess how bioenergy production may be implemented if it needs to meet these criteria. We also address sustain­ability criteria for bioenergy production as formulated by a range of institutions and relate them to the criteria for sustainable agricultural production from Section Criteria for Sustainable Farming and Sustainable Food Systems. Section How Much Bioenergy may be Pro­duced Sustainably? provides some discussion on how much agricultural bioenergy may be produced in a sus­tainable way and Section Conclusions concludes.

For a rational design of pretreatment processes is required experimental investigation of physical changes and chemical reactions that occur during pretreatment; however, due to the wide range of pretreatments and biomass available for biorefinery, the development of effective and mechanistic models can provide a large amount of information to optimize operational condi­tions. Furthermore, several key criteria regarding tech­nical, economical, and environmental considerations should be critically analyzed when adapting these tech­nologies for the nascent biorefinery industry (Sousa et al., 2009). In this section some models that particularly focus on pretreatments in the scheme of a biorefinery plant are discussed.

The most commonly developed models for the pre­treatment are kinetic models with assumptions of a first-order dependence of reaction rate on biomass com­ponents and an Arrhenius-type correlation between rate constant and temperature (Wang et al., 2011b). In view of the heterogeneous nature of the reactions involved in the pretreatment, the uses of severity factor, artificial neural network, and fuzzy inference systems, represent alterna­tive approaches for predicting the behavior of the sys­tems (Wang et al., 2011b).

A multiscale model of hydrothermal pretreatment methods, including microscale, mesoscale and macro­scale, was used to elucidate the mechanisms involved in the breakage of hemicellulose of wood (Hosseini and Shah, 2009).

A model that simulates a biorefinery plant integrating first — and second-generation ethanol production process from sugarcane, surplus bagasse and trash included a selected pretreatment method followed, or not, by a delignification step. The simulation indicated that the best results were obtained for steam explosion pretreat­ment at high solids loading and hydrolysis time between 24 and 48 h (Dias et al., 2011). Also, a mathematical model for a countercurrent shrinking-bed reactor for pretreatment/hydrolysis of hardwood cellulose predicts that dilute sulfuric acid (0.08 wt%) and with optimal adjustment of other operating parameters resulted in 80—90% yield with 2—4 wt% product concentration. This model also indicates that acid concentration and temperatures acutely affect the reactor performance in cellulose hydrolysis. In contrast, hemicellulose hydrolysis is less sensitive to acid concentration and temperature allowing broader latitude in operating conditions (Lee et al., 2000). Also, methods of optimiza­tion have been used to acid-catalyzed pretreatment pro­cess showing that the sulfuric acid concentration plays the major role during the pretreatment of areca nut husk (57% contribution) followed by the duration of operation (24.98% contribution) and solid loading (14.3% contribution) (Sasmal et al., 2011). Other authors have reported that for alkali pretreatment of cereal crop residues, the temperature had the greatest impact on sugar release, followed by alkali concentration and treat­ment time (Vancov and McIntosh, 2011). A statistical optimization method proposed variables such as tem­perature, sulfuric acid concentration and reaction time to release xylose from sugarcane bagasse as a useful means of trading off the combined effects of these three variables on total xylose recovery yields (Um and Bae, 2011). Other works considered also the reduction of the acid concentrations and reaction times to optimize the pretreatment process. This kinetic model predicted optimum conditions to pretreatment of corn stover of 150 °C, 0.6% HNO3 and 1 min of reaction time for maximal xylose, glucose and arabinose yields and min­imal yield of acetic acid and furfural (Zhang et al., 2011).

To deeply understand the factors that affect the con­version of lignocellulosic biomass to fermentable sugars, experimental results should be bridged with process simulations (Wang et al., 2011b).

Advantages of using lipases in biodiesel production are the following:

• Ability to work under different media environments includes biphasic system and monophasic system (aqueous and nonaqueous) (Mittelbach, 1990; Linko et al., 1994; Mukesh et al., 1994).

• They can be produced in bulk and are sturdy and adaptable enzymes.

• Separation is not necessary if transesterification process with lipase carried out in a packed-bed reactor.

• Proficiencies like short-chain, alcohol-tolerant and higher thermo stability of lipase make it very appropriate for use in biodiesel production (Ghaly et al., 2010).

• Thermal stability of lipases makes it possible to run the transesterification process at elevated temperatures which allows (1) increased solubility of lipids and other hydrophobic substrates in water;

(2) higher diffusion rates; (3) decreased substrate viscosities; (4) increased reactant solubilities;

(5) faster reaction rates; and (6) reduced risk of microbial contamination.

HISTORICAL BACKGROUND OF LIPASE

Over 300 years ago, triglycerides hydrolyzing enzymes have been studied well. Nearly 70 years ago, lipase’s catalysis ability and synthesis ability have been known. In 1856, Claude Bernard first revealed that pancreatic juice contains an enzyme (lipase) that hydrolyzed the insoluble oil droplets and converted them in to soluble products. In 1901 activity of microbial lipases has been observed in Bacillus prodigiosus, Bacillus pyocyaneus and Bacillus fluorescens. Serratia marcescens, Pseudomonas aeruginosa and Pseudomonas fluorescens are the best-studied lipase-producing bacteria (Fariha et al., 2006).

Lipase-catalyzed biodiesel production was reported first by Mittelbach (1990). Depending upon the speci­ficity, lipases are divided into three groups: (1)

1,3- specific, (2) fatty acid-specific, and (3) nonspecific. Among these three, 1,3-specific lipases discharge fatty acids from positions 1 and 3 of a glyceride and hydro­lyze ester bonds in these positions (Antczak et al., 2009; Ribeiro et al., 2011).

Microalgae are a diverse group of photosynthetic organisms whose systematics is based on the kinds and combinations of photosynthetic pigments present in different species. They can grow in diverse environ­mental conditions, and are able to produce a wide range of chemical products with applications in feed, food, nutritional, cosmetic and pharmaceutical indus­tries. These are primitive organisms with a simple cellular structure and a large surface to volume body ratio, which gives them the ability to take up a large amount of nutrients. While the mechanism of photo­synthesis in microalgae is similar to that of higher plants, they have the ability to capture solar energy with an efficiency of 10—50 times higher than that of terrestrial plants (Li et al., 2008). Moreover, because the cells grow in aqueous suspension, they have more efficient access to water, CO2 and other nutri­ents. For these reasons, microalgae are capable of pro­ducing more amount of oil per unit area of land in comparison to that of all other known oil-producing crops (Chisti, 2007; Haag, 2007). The per hectare yield of microalgal oil has been projected to be 58,700—136,900 l/year depending upon the oil content of algae, which is about 10—20 times higher than the best oil producing crop, i. e. palm (59501/ha year, Chisti, 2007). The most acclaimed energy crop, i. e. Jatropha has been estimated to produce only 1892 l/ha year. More importantly, due to being aquatic in nature, algae do not compete for arable land for their cultiva­tion; they can be grown in freshwater or saline, and salt concentrations up to twice that of seawater can be used effectively for few species (Aresta et al., 2005; Brown and Zeiler, 1993). The utilization of wastewaters that are rich in nitrogen and phosphorus may bring about remarkable advantages by providing N and P nutrients for growing microalgae, while removing N and P from the wastewaters (Mallick,

2002) . This implies that algae need not compete with other users for freshwater (Campbell, 2008). On top of these advantages, microalgae grow even better when fed with extra carbon dioxide, the main GHG. If so, these tiny organisms can fix CO2 from power sta­tions and other industrial plants, thereby cleaning up the greenhouse problem. Each ton of algae produced consumes about 1.8 ton of CO2 (Chisti, 2007). Thus, the integrated efforts to cleanup industrial flue gas with microalgal culture by combining it with waste­water treatment will significantly enhance the envi­ronmental and economical benefits of the technology for biodiesel production by minimizing the additional cost of nutrients and saving the precious freshwater resources.

A screw press or a centrifuge can separate the diges — tate into a solid and a liquid fraction. Recycling of the solid fraction will increase the solid content of the digesters further. The recycled solid fraction will yield between 30 and 100 l/kg VS depending on the pH and protein content (Balsari et al., 2010).

Disposal of the liquid fraction into a sewer or into sur­face waters requires the removal of phosphate and nitro­gen. Phosphate can be separated by the addition of magnesium and precipitation of magnesium ammonium phosphate (struvite). The production of ammonium requires about 30 MJ/kg. The ammonium can be recov­ered by adding extra phosphate and magnesium to the effluent.

Tuerker and Celen (2007) give a cost for chemicals of

7.7 $/kg N removed (price level of 2001) for magnesium chloride and phosphoric acid and sodium hydroxide. About a third of the cost is for the sodium hydroxide necessary for the adjustment of pH. Struvite is not a conventional fertilizer and the price is quite specula­tive, but it should be at least the value of the phosphate 2.9 $/kg N.

Dvorak et al., 2011 heat the liquid fraction of digestate up to 70 °C and use aeration for the removal of CO2. This results in an increase in the pH to nearly 10 and a shift of the ammonia ions to gaseous ammonia. The diet of dairy cows is rich in calcium. Consequently the phosphate in the digestate is in the form of insoluble calcium phos­phate particles. CO2 bubbles keep these particles in sus­pension. Elimination of the CO2 results in settling of the particles in a quiescent tank. The system removes 70% of ammonia and 80% of phosphate. End products are a so­lution (35%) of ammonium sulfate and phosphate — containing solids.

Karakashev et al. (2005) came to the conclusion that microfiltration is unsuited for treatment of digested pig manure due to membrane clogging. They developed a method at laboratory scale to clean the supernatant after decanting-centrifuging. It involves a up-flow anaerobic sludge blanket reactor, precipitation of magnesium-ammonia-phosphate (struvite) by adding magnesium oxide, partial aeration and ammonium removal by anaerobic ammonia-oxidizing bacteria.

Phosphate and nitrogen can also be concentrated by removing water from the liquid fraction. Waste heat (50—70 °C) from electricity generation removes only fraction of the water in a single pass. Up to three passes are possible.

Distillation with vapor recompression has been tried (Melse et al. 2005). The electric energy consumption was about 0.3MJe/kg water removed. Technical and economical reasons led to abandonment of the process.

The Biorek (Preez et al., 2005) process uses a two-step filtration and reverse osmosis process to increase the solids content. One project in the Netherlands was stopped due to operational difficulties.

The 20 MWe biogas plant in Penkum, Germany, uses a decanter followed by a swinging screen for the removal of solids and evaporation and reverse osmosis for the removal of salts (Herbes, 2010).

There are a number of key features for the effective pretreatment of lignocellulosic biomass. The pretreat­ment process should have a low capital and operational cost. It should be effective on a wide range and loading of lignocellulosic feedstocks and should result in the recov­ery of most of the lignocellulosic components in a usable form in separate fractions. The need for preparation/ handling or preconditioning steps prior to pretreatment such as size reduction should be minimized. It should produce no or limited amounts of sugar and lignin degradation products that inhibit the growth of fermen­tative microorganisms or the action of hydrolytic en­zymes, and it should have a low energy demand or be performed in a manner that energy invested could be used for other purposes such as secondary heating (Agbor et al., 2011). The ideal pretreatment process produces a disrupted, hydrated substrate that is easily hydrolyzed and optimized to accommodate the requirements of subsequent conversion steps, e. g. the formation of sugar degradation products and fermenta­tion inhibitors is avoided, inorganic materials is mini­mized and/or optimized separation of the main constituents lignin, cellulose and hemicellulose is achieved. Pretreatment technologies are always a combi­nation of physical/physicochemical and chemical steps. Physical pretreatment involves the size reduction by cut­ting, milling or grinding. Smaller particle sizes result in improved hydrolysis/solvation because of increased sur — face/volume ratio of the substrate resulting in improved mass transfer rates. Barakat and coworkers reviewed the dry fractionation of lignocellulosic biomass. They concluded that particle sizes must be reduced to 0.5—2 mm in order to decrease heat and mass transfer limitations and to reach a well-accepted level of digest­ibility. However, currently mechanical size reduction steps are not cost-effective because of too high energy demands of dry grinding operations; therefore, innova­tive grinding and milling processes or combinations of mechanical size reduction with others pretreatments are still required (Barakat et al., 2013). Table 17.5 highlights the advantages and disadvantages of the different pretreatment technologies that will be dis­cussed in more detail below. The importance of high — solids loadings in biomass pretreatment has recently been reviewed (Modenbach and Nokes, 2012).

Steam Explosion

Steam explosion pretreatment is one of the most commonly used pretreatment technologies, as it uses a combination of physical and chemical techniques in order to open up and partly break down the structure of lignocellulosic biomass. The steam-explosion pre­treatment is a (autocatalytic) hydrothermal process, which subjects the biomass to high pressures and tem­peratures for a short duration of time after which the system is rapidly depressurized, causing a disrupting of the three-dimensional structure of the biomass. The disruption causes a partial solubilization of the hemicel — lulose and lignin fraction of the biomass subsequently increasing the accessibility of the cellulose to the hydro­lytic enzymes. Particle size is a major contributing factor on the effectiveness of the process, and it has been seen that relatively large particle sizes have been able to yield maximum sugar concentrations. This is a promising finding, as decreasing the particle sizes of the material requires further mechanical processing of the raw material driving up the production costs (Brodeur et al., 2011). Temperatures ranging from 190 to 270 °C have been used with residence times of 1—10 min, respectively. The addition of acidic catalysts has been explored in minor amounts in order to improve hemicel — lulose hydrolysis during the pretreatment and cellulose

digestibility further on in the process. In addition, a reduction of inhibitory compounds formed is seen. The addition of acidic catalysts causes the hydrolysis of acetyl groups into acetic acid. The physical pretreat­ment is realized during the rapid decompression of the system. This causes a rapid expansion by vaporization of the saturated water within the lignocellulosic biomass; this results in the breakage of the molecular linkages, and leads to a lignocellulosic matrix very sus­ceptible to enzymatic hydrolysis. The steam-explosion pretreatment process has been a proven technique for the pretreatment of different biomass feedstocks. It is able to generate complete sugar recovery while utilizing a low capital investment and low environmental im­pacts concerning the chemicals and conditions being implemented and has a higher potential for optimiza­tion and efficiency (Brodeur et al., 2011). The steam — explosion pretreatment has been demonstrated using a wide range of biomass sources including poplar chips, olive tree residues, wheat straw and corn stover. Howev­er, some disadvantages can be seen when using this process. Dilute acids are needed when using softwoods or even when increased yields are warranted for lower acetylated feedstocks. However, the addition of acids comes at a cost because it results in elevated equipment requirements and the higher formation of degradation products such as furfural and 5-hydroxymethyl furfural (HMF), which is sometimes detrimental for subsequent fermentations. In addition salts are formed because the liquors need to be neutralized. These salts need subse­quently to be separated from the system and disposed. In the past the focus of many pretreatment technologies was the optimization of the cellulose recovery and subsequent conversion. However, cellulose content of biomass is seldom above 45% making the economics challenging especially because of rising feedstock and energy costs. Therefore, research and developments are now moving into the direction of complete utiliza­tion of the entire lignocellulosic biomass.

The growth of the lignin business in this century depends, for the most part, on the ability of chemists and materials scientists to develop novel original appli­cations and processes for lignin valorization. From 1990 to 2010, a number of novel lignin applications have been proposed and described in the scientific and patent liter­ature. Some of these novel applications have been piloted or demonstrated at larger than laboratory scale. For instance, the production of lignin-based carbon fiber (LCF) (Baker and Rials, 2013) is one of the brightest ex­amples of successful lignin upgrading technology which have been scaled up to pilot scale. Another example of nontraditional lignin applications which is being commercialized with a market potential comparable to carbon fiber is the use of upgraded technical lignins in polymers, in particular, in thermoplastic, thermoset, and composite applications. Not far in the future, we will wit­ness the rise of processes aiming at depolymerization of lignins to produce valuable oxygenated aromatic com­pounds, and possibly olefins too, in replacement of petro­chemicals. This section briefly summarizes these three relevant examples of emerging nontraditional lignin applications which in our opinion present the largest potential, in terms of volume and value, for commercial­ization of technical lignins in the future.

Among all the emerging lignin applications, the manufacturing of LCFs is perhaps the one with the largest market potential in terms of value. This dream finds its beginning in the early 1960s, in Japan, when a group led by Dr. Sugio Otani from the Department of Chemistry, Faculty of Technology, Gunma University developed a technology to turn lignin into carbonized fi­bers (Otani et al., 1969). The Kajima Corporation, a giant Japanese construction company, working together with Dr. Otani, had developed, on the basis of this early invention, a lignin—fiber-based reinforced cement with superior strength properties. Since the early work of Dr. Otani, the quest to develop commercially viable LCF applications has been focused on achieving lignin and lignin blends with properties enabling high rate of fiber spinning (>2000 m/s), yielding scalable and fast conversion technologies, as well as high yield. However, regardless of all the efforts made so far by multiple research groups worldwide, in particular, by the Carbon Fiber Composites Consortium (Oak Ridge National Lab­oratory), the development of high-quality structural LCF has proved to be very challenging. The main chal­lenge remains achieving the required LCF engineering properties imposed by existing carbon fiber spinning technologies. In particular, the best LCF prototypes have shown a relative low fiber tensile strength (~ 1 GPa) and low fiber elastic modulus (<100 GPa) while most structural applications (automotive, sport goods, wind turbines, and aerospace applications) require ten­sile strengths well above 1 GPa and elastic moduli over 100 GPa. Attempts to overcome these limitations have been made where lignin is blended with synthetic poly­mers, such as acrylonitrile, and copolymerized to yield hybrid LCF with acceptable tensile strengths (Mara — dura et al., 2012). In addition to the LCF mechanical limitations, the relatively high cost can be pointed out as another limiting factor deterring LCF commerciali­zation. Owing to these hurdles, the focus of LCF R&D efforts has been recently shifting from structural appli­cations toward functional uses where LCF seem better suited. Examples of functional uses are high — temperature insulating materials, CO2 and other gases capture sorbents, controlled adsorption and release of macromolecules, and capacitors. On the cost reduction side, besides the improvement of technical lignins as a raw material, so their upgrading costs can be mini­mized, there is the potential to realize cost reductions by way of alternative fiber spinning methods that are not bound by the stringent technical requirements needed for the traditional melt-spinning of carbon fiber precursors (Baker and Rials, 2013). In addition to cost reduction and attempts to improve LCF mechanical properties, the development of novel technologies to further upgrade lignin will be required to meet the demanding industrial standards.

Valorization of lignin can be achieved also via its incorporation in polymeric materials, in particular, in composite materials containing thermosets, such as, phenol—formaldehyde, urea—formaldehyde, and epoxy resins (Zhao et al., 2001; Mankar et al., 2012; Wang et al., 2012; Yin and Di, 2012), and, thermoplastics, such as lignin blends for extrusion applications with polyesters (Li and Sarkanen, 2002), polyamides (Nitz et al., 2001), polyacrylonitrile (Seydibeyoglu, 2012), and polyethyl — enes (Casenave et al., 2009). Technical lignins find also applications in structural materials such as polyure­thanes which are recognized as one of the most versatile classes of polymeric materials and they show good compatibility with lignin given the presence of both ar­omatic and aliphatic hydroxyl groups within the lignin structure (Cateto et al., 2010; Faria et al., 2012). The cornerstone of a viable wide incorporation of technical lignins in synthetic polymer blends is their compatibility with the chemical matrices. The compatibility between lignin and synthetic polymers, often more hydrophobic than lignin, can be achieved, for instance, via chemical modification of lignin through esterification (Li and Sar- kanen, 2002) or during lignin production by modifying the reaction conditions.

Finally, the depolymerization of lignins to produce valuable oxygenated aromatic compounds is an applica­tion which, if successful, will consume most of the tech­nical lignin supply in the future. However, this goal faces, perhaps, one of the most challenging lignin tech­nological barriers (Holladay et al., 2007). Multiple approaches have been attempted to achieve the evasive lignin depolymerization target. The thermochemical methods including pyrolysis (thermolysis), gasification, hydrogenolysis, chemical oxidation, and hydrolysis un­der supercritical conditions are the major methods stud­ied with regard to lignin depolymerization (Pandey and Kim, 2011). The biochemical route, due to its relative low capital required for deployment and low-energy opera­tion, has been extensively researched as a way of selec­tively depolymerizing lignin but its high cost and relatively long reaction time continues to be a barrier for commercialization (Chen et al., 2012a, b).

CONCLUSIONS

The low value of technical lignins as a fuel has been evidenced in this review. However, new technological developments for smart utilization of technical lignins can lead to much higher value market opportunities, possibly with lignin-based chemicals superior to petrochemicals as it was illustrated in the case of lignin-based hybrid resins. Further technological ad­vancements in the upgrading and optimization of tech­nical lignins for materials applications is currently constrained by our ability to better understand their chemical structure and reactivity as well as by our capac­ity to efficiently purify and adapt them to the needs of the demanding chemical industry. Advancements in devel­oping comprehensive and validated lignin analytical methods applied to purposeful lignin applications will help with mitigating the existing technological hurdles. Regrettably, the efforts made toward developing lignin as a chemical feedstock remain very modest compared, for instance, to those made toward the utilization of other biomass components such as cellulose and hemicellu — lose. The refining of complex raw chemical streams into building blocks is a matured technology best exemplified by the petroleum and gas industries. Therefore, the development of lignin-refining technologies into valu­able chemicals should also be possible in the near future if adequate resources are directed toward this goal.

Genetic Modifications

Most important genetic modifications applied to cya­nobacteria in order to improve their H2 production capacity have already been discussed above. Therefore, just a few recent and important findings, will be mentioned below.

Introducing Foreign Enzymes and Semiartificial Systems

Most attempts of heterologous expression of geneti­cally modified hydrogenase in cyanobacteria have not been successful due to the complexity of transcriptional regulation and maturation of the hydrogenases. A successful heterologous expression of Fd-dependent hydrogenase from Clostridium in the Synechococcus PCC7942 was demonstrated and resulted in about three­fold higher H2 production activity compared to the wild type (Asada et al., 2000). More recent studies related to the identification and characterization of the genes involved in maturation and regulation of the [Fe—Fe]- and [Ni—Fe]-hydrogenases and nitrogenases (Rubio and Ludden, 2005) as well as the development of heter­ologously expressed enzyme systems have opened new opportunities. In an attempt to engineer an organism, which can produce H2 even under aerobic conditions, the [Fe—Fe]-hydrogenase operon from Shewanella onei — densis MR-1 containing all maturation genes was suc­cessfully expressed in the heterocysts of Anabaena PCC 7120 (Gartner et al., 2012). Active [Fe—Fe]-hydrogenase was detected in aerobically grown Anabaena PCC 7120 under diazotropic growth. Despite significantly higher turnover number of the [Fe—Fe]-hydrogenase, in situ H2 production rate was only about 20% of that by nitro- genases (Masukawa et al., 2010). One of the reasons for such low activity might be that Fd in Anabaena is not an effective electron donor to the [Fe—Fe]-hydrogenase.

Some hydrogenases, like the [Ni—Fe]-hydrogenase of Ralstonia eutropha are able to perform H2 cycling in the presence of ambient O2. Employing this interesting property of the Ralstonia hydrogenase, a [Ni—Fe]- hydrogenase (Hox) from Ralstonia has been fused with the extrinsic PsaE subunit, which is located at the acceptor site of PS I, by genetic engineering. The resulted Hox/PsaE fusion exhibited in vitro self-assembly with a cyanobacterial PSI lacking the PsaE subunit and light — driven H2 was evolved (0.58 mmol H2 mg/Chl h, Ihara et al., 2006a). In another study, Cytochrome c3 (Cyt c3) from Desulfovibrio vulgaris was chemically cross-linked to PsaE protein and the Cyt c3/PsaE complex was rebound to a PsaE-free PSI complex and introduced to a solution containing the D. vulgaris [Ni—Fe]- hydrogenase enzyme (Ihara et al., 2006b). When illumi­nated, light-induced H2 generation was observed at a maximum rate of 0.30 mmol H2 mg/Chl h in the presence of Fd and FNR, physiological acceptors of PSI. These results suggest that the Cyt c3/PSI complex may produce H2 in vivo. However, the observed in vitro H2 production rate was quite low, most probably due to poor electron transfer coupling between PSI and the hy — drogenase in solution.

A further coupling approach has been applied where the FB cluster of PSI and catalytic nanoparticle surface have been covalently bound via a "molecular wire". Upon illumination, this semiartificial system generated up to 70 mol H2 PSI/mol min (Grimme et al., 2008, 2009). In this innovative system, covalent binding between Fb (an electron cofactor of PSI) and a "molecu­lar wire" (catalytic nanoparticle surface) enables high rates of H2 evolution.

Finally, the direct coupling of photosynthesis and H2 production has also been performed on a gold surface (Krassen et al., 2009). For stable connection of the [Ni—Fe]-hydrogenase to PSI, the extrinsic PsaE subunit was fused to the electron-transferring subunit of the membrane-bound [Ni—Fe]-hydrogenase by genetic engineering. The resulting Hox/PsaE protein was puri­fied and incubated with isolated PSI from Synechocystis sp. PCC 6803 lacking the PsaE subunit (PSIDPsaE). PSIDPsaE and HoxPsaE were assembled on a gold sur­face and electrons provided by the gold electrode were transferred to PSI with the aid of the soluble electron car­rier N-methylphenazonium methyl sulfate. Upon light illumination the hydrogenase-PSI hybrid system demonstrated H2 production a rate of 4500 mol H2/min mol (Krassen et al., 2009).

Charles Hyland*, Ajit K. Sarmah

Department of Civil & Environmental Engineering, The University of Auckland, Auckland, New Zealand
Corresponding author email: chyl531@aucklanduni. ac. nz

OUTLINE

Introduction 435

Properties of Biochar 436

Chemical and Physical 436

Microbiological Effects and Synergisms 437

Utilization of Biochar for Environmental Quality 438

Carbon Sequestration 438

Sorption of Plant Nutrients and Other Pollutants 438

Soil Greenhouse Gas Emissions 439

Soil-Specific Biochar Design 440

Postpyrolysis Indirect Application of Biochar 440

Water Filtration 440

Soil Nutrient Reclamation 441

Biochar As Container Growth Medium and Container 441

Conclusions, Knowledge Gaps, and Research Needs 443

References 444

INTRODUCTION

In its simplest material context, biochar is incom­pletely combusted organic matter that is applied to soil; however, this material has attracted considerable attention in recent years due to goals that may be achiev­able through its production and end use. In this chapter, advances and innovations in biochar production and in­termediate uses are presented and discussed. A consid­erable proportion of the world’s natural soil organic carbon content comprises black carbon, a pool that is resistant to microbial degradation and was deposited from historic fires (Krull et al., 2008). Augmenting this nearly ubiquitous pool with additional recalcitrant car­bon has recently been the subject of much scientific research focused on soil improvement and carbon (C) sequestration, which has garnered notable support from some of the world’s most well-known climate
scientists and environmental advocates, such as Al Gore (Gore, 2009), Tim Flannery (Flannery, 2009), James Hansen (Hansen et al., 2008), and James Lovelock (Lovelock, 2009). Biochar is a form of anthropogenic recalcitrant carbon produced for the purpose of applica­tion to soil that draws inspiration from the practices of precolonial Native Amazonians who transformed some of the world’s poorest soils into extremely fertile soils that remain productive into the present, centuries after their production ceased (Lehmann, 2008). This rediscovered concept represents both a stable form of C-rich soil organic matter and a means to ameliorate degraded soils into fertile soils (Lehmann et al., 2006). C sequestration is achievable through a range of thermo­chemical conversion processes (Table 25.1), including pyrolysis, a process that produces biochar as a by­product. Recalcitrant carbon, including particulates emitted by fossil fuel combustion, may represent up to

Bioenergy Research: Advances and Applications http://dx. doi. org/10.1016/B978-0-444-59561-4.00025-5

30% of all soil C globally (Schmidt et al., 2001). Woolf et al. (2010) reported that by increasing biochar produc­tion net anthropogenic greenhouse gas (GHG) emissions could be reduced by as much as 12% through the substi­tution of pyrolysis oils and gases for fossil fuels.

Recent reviews and studies have highlighted the potential environmental as well as agronomic benefits of biochar (Kookana et al., 2011). While many of these potential environmental benefits of biochar are largely known through glasshouse and field trials, there have been findings that also showed negative impacts of biochar utilization as soil amendment (Clough et al., 2010). Given that biochar can be produced from a variety of feedstocks, and quality of biochar is dependent on the type of feedstock and pyrolysis conditions, not all bio­chars are made equal.

The pyrolysis bioenergy industry is currently in the early stages of commercialization (Downie and van Zwieten, 2013) and is growing slowly due to the vast heterogeneity of biochar feedstocks, production methods, and end uses (Jahirul et al., 2012). The eco­nomic viability of commercial pyrolysis is likely to in­crease in tandem with the price of C credits assigned to biochar (Laird et al., 2009). Due to the characteristic persistence of biochar after incorporation into soil, pre­requisite testing of biochars is crucial in order to avoid production of biochars that either form toxic com­pounds, such as polycyclic aromatic hydrocarbons formed as a result of pyrolysis (Verheijen et al., 2010; Kloss et al., 2012), or retain toxins, such as heavy metals, that were present in a feedstock, such as municipal solid waste or sewage sludge (Sparkes and Stoutjesdijk, 2011; Kookana et al., 2011). Minimal predictive capability currently exists in relation to biochar performance in the field (Sohi et al., 2010). Field trials, preceded by initial laboratory and Glasshouse testing, are the preferred biochar efficacy evaluation methods for con­ventional biochar, as well as novel biochar uses that are currently being investigated. Thorough research must be conducted in order to ensure that biochars suited to address specific issues are selected from the diverse array of biochars that are currently being pro­duced, in order to avoid potential socioeconomic and environmental damage (Barrow, 2012).

Rather than applying arbitrarily chosen biochars and passively observing their effects on the environment, a forefront of biochar research is the custom design of bio­char for specific targeted end uses. Biochar is commonly referred to as a soil conditioner (Lehmann et al., 2006) in order to contextually differentiate the material from direct fertilizers. Given this, it rationally follows that bio­chars and their deployment methods be engineered to enhance the soil and the surrounding environment in a contextually desirable manner. For example, if it is known that biochar efficacy is increased once the biochar surfaces are coated with clay minerals in a given soil after a period of time, then producing biochars that already possess this quality at the time of incorporation into soil would be a significant advancement. Evaluating intermediate uses for biochar, that is functions that bio­char can perform after production and before soil incor­poration, may simultaneously improve the efficacy of the biochar and reduce preexistent downstream and atmospheric pollution. An example of an intermediate use discussed in this chapter is the employment of bio­char contained within permeable mesh bags for the filtration of runoff and leachate water.

Current bioenergy resources consist of residues from forestry and agriculture, various organic waste streams and dedicated biomass production from pasture land, wood plantations and sugar cane (Figure 2.2). At pre­sent, the main biomass feedstocks for electricity and heat generation are forestry and agricultural residues and municipal waste in cogeneration and cofiring power plants. In the longer term, lignocellulosic crops could provide bioenergy resources for second-generation bio­fuels, which are considered more sustainable, provide land use opportunities and will reduce the competition with food crops (http://www. ga. gov. au/image_cache/ GA16706.pdf).

Major feedstock sources for future biofuel production are likely to be high biomass producing plant species such as poplar, pine, switchgrass, sorghum maize, Miscanthus, hemp, Jatropha, willow and cassava. With
growing interest in the utilization of plant biomass for the production of ethanol and other biofuels, the use of plant species as biofuel feedstocks has become a focal point in research. Due to concerns about diverting grain and seed from human food and livestock feed to biofuel feedstock production, emphasis has shifted to the use of lignocellulose-derived biofuel production, and research is now directed at improving not only lignocellulosic yield but also quality traits in these species (Banerjee, 2011; Mueller et al., 2011; Tyner, 2010).

A long-term opportunity exists to produce fuels from nonedible lignocellulosic biomass from plants (Heather and Somerville, 2012). Sugarcane, energy cane, elephant grass, switchgrass, and Miscanthus have intrinsically higher light, water and nitrogen use efficiency and are fast-growing biomass/crops for bioenergy work pro­gram. Work on perennial grasses such as switchgrass (Panicum spp.), prairie cordgrass (Spartina spp.), big bluestem (Andropogon spp.), little bluestem (Schizachy — rium spp.) and others could produce significant biomass in a variety of biomass throughout the northern plains and southeastern grasslands in the United States (Gonzalez-Hernandez et al., 2009). Woody biomass can be harvested sustainably for lumber and paper and may, therefore, provide biofuel feedstock for some re­gions (Malmsheimer et al., 2011). Table 2.5 summarizes the countrywise contribution of current biofuel yield from different feedstocks.

As mentioned previously, biomass energy can come from numerous sources and produce several types of fuels. Ethanol is typically produced from biomass

Wood industry

residues 5%

Recovered

wood 6%

Municipal solid

waste and landfill gas

high in carbohydrates (sugar, starch and cellulose) dur­ing a fermentation process. Recent developments in fermentation processes now allow almost any plant type to be used to produce ethanol. The most prom­ising natural oils, such as rapeseed oil, have been used to produce biodiesel, which performs much like petroleum-derived diesel fuel. Apart from agricultural, forestry and other by-products, the main source of lignocellulosic biomass for second-generation biofuels is likely to be from "dedicated biomass feedstocks", such as certain perennial grass and forest tree species. Genomics, genetic modifications and other biotechnol­ogies are all being investigated as tools to produce plants with desirable characteristics for second — generation biofuel production, for example, plants that produce less lignin (a compound that cannot be fermented into liquid biofuel), plants that produce en­zymes themselves for cellulose and/or lignin degrada­tion, or plants that produce increased cellulose or overall biomass yields. Grass, leaves, agri crops, agri­crop residues and currently available nonfood plant biomass are the dominant source of lignocellulosic materials (Carpita, 2012; Ambavaram et al., 2011;

Abramson et al., 2010; Davison et al., 2006; Nguyen et al., 1999, 2000).

Bioenergy resources used in current biofuels develop­ment programs, potential future resources and the related bioenergy outputs are summarized in Table 2.6. Bioenergy resources are difficult to estimate due to their multiple and competing uses. Production statistics exist for current commodities such as grain, sugar, pulp wood and saw logs; however, these commodities are currently largely committed to food, animal feed and materials markets. Potential feedstocks for the future include modified strains of existing crops, new tree crops and algae. There are many factors to be taken into ac­count for each bioenergy resource, such as moisture con­tent, resource location and distribution, and type of conversion process that is most suitable. Different sour­ces of biomass require very different production systems and therefore a variety of sustainability issues can arise. These range from very positive benefits (e. g. use of waste material, or growing woody biomass on degraded agricultural land) through to large-scale diversion of high-input agricultural food crops for bio­fuels (O’Connell et al., 2009).