During thermochemical conversion, alkali metals (Na, K, Mg, P, Ca) present in the ash react with silica — originating both from the biomass itself and from soil introduced dur­ing harvesting — to produce a sticky, mobile liquid phase that can contribute to slag­ging, deposition, and corrosion of process equipment. As noted above, water leaching and fuel additives can be used to reduce the damaging effects of ash components, including alkali metals.

1.2.2 Carbohydrate/Lignin Ratio

In biological processing, carbohydrate present in cellulose (and potentially hemicellulose) is converted to fuels and/or chemicals, while the lignin fraction remains unaffected. Fur­thermore, the recalcitrance of cellulosic biomass to bioconversion typically increases with increasing lignin content, requiring more severe pretreatment, which decreases process efficiency. Bioconversion processes, therefore, favor feedstocks with high carbohydrate to lignin ratios. Representative cellulose, hemicellulose, and lignin values for many of the biomass crops considered in subsequent chapters are listed in Table 1.1.

1.3 Desirable Traits and Potential Supply of Cellulosic Energy Crops

Given the world’s finite land resource, the most important trait for cellulosic energy crops is productivity — the annual dry matter produced per unit land area. As listed in Table 1.1, productivity of the crops considered in this book ranges from 0.1 to 1.75 Mg/ha/yr (dry basis) for wheat straw, to as high as 44 Mg/ha/yr (dry basis) for miscanthus. The best energy crops will also have few inputs and low production costs. Easily established, robust perennial crops having long life spans (e. g. >10 years) are favored over annual crops, as are those having low fertilizer, pesticide, and insecticide requirements. Native, non-invasive species that provide good habitats for wildlife are preferred.

Feedstocks used in thermochemical processing should be harvested when moisture con­tent is relatively low to minimize preliminary energy intensive drying. Low moisture is not as critical in bioconversion feedstocks, for which wet storage can sometimes be a viable option. Ideally, ash content should be low (e. g. <1%), ash melting temperatures should be high (e. g. >1500°C), with low levels of particularly damaging elements, including alkali metals, alkaline earth metals, silicon, chlorine, and sulfur.

Conventional plant breeding — which involves manipulating the genes of a species via selection and hybridization so that desired genes are packaged together in the same plant and as many detrimental genes as possible are excluded — has traditionally been used to enhance desired agronomic traits such as productivity, water use efficiency, and crop lifespan. Breeding systems have been developed, and continue to be developed, that can be used to improve virtually all plant species. The productivity of corn, for example, has more than quadrupled since the 1930s largely through conventional breeding [30]. Biomass productivity can potentially be increased even further using more sophisticated biotechnology techniques. Recent molecular and genetic studies have identified a number of regulators of plant biomass production — for example, vegetative meristem activities, cell elongation, photosynthetic efficiency, and secondary wall biosynthesis — that might be manipulated to enhance energy crop yields [31].

The potential to produce viable energy feedstocks is vast. A detailed study led by the Oak Ridge National Laboratory estimates that the United States could produce 602-1009 million dry tons annually by 2022, and 767-1305 million annual dry tons by 2030, at a price of $60 per dry ton [32]. (The low value in the range assumes a 1% annual increase in yield; the high value, a 4% annual increase.) This excludes resources that are currently being used, such as corn grain and forest products industry residues. When currently used resources are included, the total biomass estimate jumps to over one billion dry tons per year for the lower productivity case — enough to displace about half of the country’s current gasoline consumption (134 billion gallons/year) if converted to ethanol at a yield of 100 gallons/dry ton. Estimates for the global annual supply of biomass feedstocks range from 100 to 400 EJ/year — equivalent to 6 to 24 billion dry tons. If converted to ethanol, this represents 120-460% of current global gasoline consumption (338 billion gallons/year).

The study of pyrolysis is gaining increasing importance, as it is not only an independent process but is also a first step in the gasification or combustion process. Pyrolysis refers to the thermal decomposition of biomass and organic compounds in the absence of oxygen to produce biochar, oil and/or gas, depending on the temperature and reaction time. In slow pyrolysis charcoal or biochar is produced (temperature ~300°C, reaction time of hours). In fast pyrolysis (t° 400-500°C, reaction time of minutes or seconds) and flash pyrolysis (t° >700°C, reaction time of fractions of a second) mainly a liquid bio-oil is formed [49]. All pyrolysis products (char, oil, gas) can be used for the generation of heat and power. The dark-brown mobile liquid produced by fast pyrolysis can serve as an intermediate for a wide variety of applications. Clean-up, conditioning, and stabilization of the bio-oil are necessary to convert it into a product suitable for delivery to a petroleum refinery or future biorefinery, where it can be further upgraded to renewable biofuels (diesel, ethanol, bio

jet fuel) and chemicals [49, 50]. Pyrolysis offers the possibility of decoupling liquid fuel production from energy production by converting the biomass into a liquid with increased energy content that can be easily stored and transported and that has a more consistent (and specified) quality compared to the solid biomass.

Despite rapid development over the last few decades, bio-oil production through pyrolysis is still an immature technology and is not commercially feasible yet. Pyrolysis bio-oil needs to overcome many technical, economic and social barriers to compete with traditional fossil fuels [51]. In particular, the complexity of the bio-oil constitutes a big challenge.

Integrated biorefineries combine an assortment of existing technologies including those discussed earlier and other novel ones to convert biomass into biofuels, high-value bio­products, as well as heat and power. The idea behind an integrated biorefinery is to be able to produce a variety of high-value products as efficiently as the current petrochemical industry. Within the integrated biorefinery, heat and electricity are produced through con­version technologies, then used to produce high-value products, recycled to aid conversion, or sold on the commercial market (Figure 3.4). To be feasible, an integrated biorefinery must overcome a variety of challenges, including diversity of feedstocks, sustainability, and economic viability. Currently, pilot-, demonstration — and commercial-scale integrated biorefineries are operating in the United States [29].

The concept of integrated refineries has been implemented within the petroleum industry for many years. A typical oil refinery is capable of producing a variety of products, ranging from liquid petroleum gas, gasoline, jet and diesel fuels, wax, lubricants, bitumen, and petrochemicals, from which it manages production to maximize profit. This same concept also applies to integrated biorefineries. By producing a variety of products, the integrated

bioreflnery could help support a marginally viable biofuel market until it matures. The difficulties with an integrated biorefinery include guaranteeing a sustainable biomass supply that matches the desired product stream at a reasonable price. As the demand for biomass increases, the cost of acquiring material will likely increase, thus increasing pressure on the entire bioenergy industry.

With ever-increasing indications that resource use is exceeding the planet’s biocapacity [33] — largely driven by non-renewable fossil fuel consumption — it is clear that humankind must shift to sustainable practices in order for a peaceful, equitable, and thriving future to be possible. Furthermore, given mounting evidence of climate change — to the point that some say we are now living in a new geologic epoch, the Anthropocene [34] — this transformation must begin now and be completed within decades, not centuries. Indeed, it is fair to characterize this transition, moving from finite resource capital to renewable resource income, as the defining challenge of our time.

Most sustainable paths from primary resources to human needs pass through either plant biomass or renewable electricity, with biomass being the only foreseeable source of organic fuels, chemicals, and materials, as well as food. In comparison, other large-scale sustainable energy sources are most readily converted to electricity and heat. Because liquid organic fuels have a greater energy density than batteries, both today and with anticipated improvements in battery technology, it is reasonable to expect that organic fuels will meet a significant fraction of transportation energy demand for the indefinite future. This is particularly true for long-distance travel via personal vehicles and for heavy-duty applications, such as aviation and long-haul trucking, which account for more than half of global transportation energy [35]. Biofuels would, therefore, appear to be an essential component of tomorrow’s sustainable world rather than a discretionary option.

Cellulosic biomass energy potentially offers many environmental benefits that contribute to its sustainability, some of which are:

• Fossil fuel displacement.

• Lower emissions of greenhouse gases and other air pollutants.

• Enhanced soil quality.

• Reduced soil erosion.

• Reduced nutrient run-off.

• Enhanced biodiversity.

Demirbas [36], Rowe et al. [37], Arjum [38], and Skinner et al. [39] provide more detailed reviews and discussion of these and other potential benefits.

In addition to the environment, cellulosic biomass energy also has the potential to enhance energy security and rural economic development. Nations dependent upon petroleum face increasing security costs to ensure the steady supply of oil. The United States, for example, according to the RAND Corporation [40], spends about $75.5-93 billion per year — repre­senting between 12 and 15% of its current defense budget — to secure the supply and transit of oil. Furthermore, major oil supplying countries hold leverage over nations relying upon imports, as the oil producers control price stability. This directly affects foreign policy, forcing import nations to prioritize stability over values such as democracy, transparency, and human rights. Even if a country could produce 100% of the oil it uses, its consumers would still be vulnerable to global price fluctuations based on supply disruptions in unstable regions. Beyond consumerism, modern militaries invest for the long term — new airplanes, ships, and vehicles are expected to last decades. This requires alternative energy sources to be able to accommodate infrastructure that is likely to be in place for years.

In recognition of this, the United States Department of Defense has developed an alter­native fuels policy to “ensure operational military readiness, improve battle space effec­tiveness,” and increase “the ability to use multiple, reliable fuel sources [41].” Consistent with this, the US Navy has plans to deploy a “Great Green Fleet” strike group of ships and aircraft running entirely on alternative fuel blends — including cellulosic fuels — by 2016 [42]. It also has a goal of meeting 50% of its total energy consumption from alternative sources by 2020. To help enable these goals, the Navy — together with the Departments of Energy and Agriculture — signed a Memorandum of Understanding (MOU) to “assist the development and support of a sustainable commercial biofuels industry [43].” The MOU calls for $510 million in funding over three years to develop advanced biofuels that meet military specifications, are price competitively with petroleum, are at geographically diverse locations with ready market access, and have no significant impact on the food supply.

A cellulosic biofuels industry, by generating demand for agricultural products, has the potential to significantly increase employment in rural areas in sectors ranging from farming to feedstock transportation to plant construction and operation. Workers would be required in a variety of occupations, including: scientists and engineers conducting research and development; construction workers building plants and maintaining infrastructure; agricul­tural workers growing and harvesting energy crops; plant workers processing feedstocks into fuel; and sales workers selling the biofuels. Brazil’s sugar/ethanol industry directly employs about 489 000 workers, with an additional 511 000 workers engaged in supporting agricultural activities [44]; the United States corn ethanol industry directly employs about 400 000 [45]. A study forecasting the impact of advanced biofuels on the US economy estimates that the industry could create over 800 000 jobs by 2022 [46].

Cellulosic biofuels also have the potential to promote rural economic activity within developing nations and improve the lives of the world’s poor. Farmers would have increased demand for their products, including crop residues from existing crops, and employ addi­tional workers to produce the energy feedstocks. They would also be able to make use of degraded or marginal land not suitable for food production. Care must be taken, how­ever, to include small landholders in the sector’s development and to adequately invest in local workforce training for feedstock production, production facilities construction, and process operation. In addition, to the extent possible, the sector should be developed around existing industries, such as sugarcane processing, to lower investment barriers [47]. Also, selection of feedstock supply chains that do not compromise food security is critical. Signif­icant potential exists to actually enhance food security through bioenergy production — by using inedible crops grown on marginal land, for example, or integrating production of food, animal feed, and bioenergy. One can envision many benefits that might be realized: employ­ment and development of marketable skills; introduction of agricultural infrastructure and knowledge; energy democratization, self-sufficiency, and availability for agricultural pro­cessing; and an economically rewarding way to restore degraded land. Bioenergy could potentially improve both food security and economic security for the rural poor [48].

Such benefits, however, are by no means guaranteed. The environmental impact of biomass energy very much depends upon how the given system is designed and imple­mented. Detractors of bioenergy have called into question its sustainability, citing a number of concerns, including:

• Food versus fuel.

• Land use change (direct and indirect).

• Water use.

• Invasive species.

• Biodiversity.

This productive debate has prompted an expanding literature analyzing and discussing the keys to “getting biofuels right,” so that the promise of sustainable bioenergy can be realized [49-51]. To minimize both competition with food production and land use change effects, multiple classes of feedstocks are available, including energy crops grown on abandoned agricultural lands; food crop residues such as corn stover and wheat straw; sustainably harvested forest residues; double crops grown between the summer growing seasons of conventional row crops; mixed cropping systems in which food and energy crops are grown simultaneously; municipal and industrial wastes; and harvesting invasive species for bioenergy [49,50,52-54]. Water use can be minimized by selecting crops having low irrigation requirements, by using non-potable sources such as wastewater or high-saline water for any necessary irrigation [55,56], and using subsurface drip irrigation to minimize evaporative losses [57]. The potential for non-native energy crops becoming invasive can be limited by proper preliminary risk assessment, including test plots [58], regular monitoring and stewardship programs [59], and by using sterile plant varieties [60]. The impact of a given energy crop upon biodiversity depends strongly on specific regional circumstances, the type of land and land use shifts involved, and the associated management practices [61]. Herbaceous perennial crops, in particular, appear to be capable of providing suitable habitats for a variety of species, especially with careful attention to crop placement and when mixed cultures are used [62-65]. By incorporating many of the above strategies, Dale et al. [51] calculated that, using the 114 million hectares of cropland currently allocated in the United States for animal feed, corn ethanol, and exports, 400 billion liters of cellulosic ethanol (80% of current gasoline demand) could be made — all while producing the same amount of food. In summarizing their findings, the authors write:

Our analysis shows that the U. S. can produce very large amounts of biofuels, maintain domestic

food supplies, continue our contribution to international food supplies, increase soil fertility,

and significantly reduce GHGs. If so, then integrating biofuel production with animal feed

production may also be a pathway available to many other countries. Resolving the apparent “food versus fuel” conflict seems to be more a matter of making the right choices rather than hard resource and technical constraints. If we so choose, we can quite readily adapt our agricultural system to produce food, animal feed, and sustainable biofuels.

Any human activity involving new technology can potentially be harmful if not thoughtfully planned and appropriately conducted. The early-generation Altamont Pass wind farm in California, for example, unwittingly located on a major bird migratory route, results in thousands of bird deaths every year [66]. To remedy the problem, the farm’s owners are installing new, less destructive turbines and shutting down a significant fraction of the turbines during the migration season [67]. In the case of cellulosic biomass, if care is taken to address the key concerns noted above, the resource could very likely contribute substantially — indeed, uniquely and essentially, by accommodating energy services not easily met by other means — towards achieving a sustainable global energy future. Kline et al. [50] succinctly capture the promise of this vision:

When biofuel crops are grown in appropriate places and under sustainable conditions, they offer a host of benefits: reduced fossil fuel use; diversified fuel supplies; increased employment; decreased greenhouse gas emissions; enhanced habitat for wildlife; improved soil and water quality; and more stable global land use, thereby reducing pressure to clear new land.

This book — through detailed consideration of cellulosic energy crop production; the logis­tics of feedstock harvest, storage, and transport; and commercial deployment that is mindful of economic, environmental, and social concerns — seeks to disseminate knowledge that can help make large-scale, sustainable bioenergy a reality.

Gasification is a process that converts organic or fossil based carbonaceous materials into carbon monoxide, hydrogen, carbon dioxide and methane. This is achieved by reacting the material at high temperatures (>700°C), without combustion, with a controlled amount of oxygen and/or steam. The gasification processes may be distinguished according to the gasification agent used. When biomass is heated with no oxygen or only about one-third of the oxygen needed for efficient combustion, it gasifies to a mixture of primarily carbon monoxide and hydrogen — called synthesis gas or syngas (typically 40% CO, 40% H2, 3% CH4 and 17% CO2, dry basis), which can be used to make methanol, ammonia and diesel fuel with known commercial catalytic processes. When air is the oxidant, nitrogen accounts for about half of the product gas. This dilutes the concentration of hydrogen and carbon monoxide gases, resulting in a low-energy fuel gas or producer gas (typically 22% CO, 18% H2, 3% CH4, 6% CO2 and 51% N2) [16].

After leaving the gasifler, the product gas has to be cleaned and, depending on further pro­cessing steps, upgraded. The reasons for gas cleaning are to prevent corrosion, erosion and deposits in the process lines as well as to prevent poisoning of catalysts. Contaminants such as tars and inorganic components (halides, alkalis, ash) present in the syngas can deactivate the catalysts and must be removed prior to catalytic conversion [52]. Upgrading encom­passes modification of the CO/H2 ratio or removal of inert gas fractions, mainly CO2 [16].

Starting from the cleaned and upgraded synthesis gas, several fuel processing pathways are possible: the application of (thermo)chemical processes such as Fischer-Tropsch (pro­viding diesel/gas like biofuel) or methanol synthesis as well as biotechnological processing towards alcohols is possible [16].

The syngas produced by gasification of biomass can be converted into a large number of organic compounds that are useful as chemical feedstocks, fuels and solvents (Figure 2.4). Collectively, the process of converting CO and H2 mixtures to liquid hydrocarbons over a transition metal catalyst has become known as the Fischer-Tropsch (FT) synthesis, invented in the 1920s by the German engineers Franz Fischer and Hans Tropsch. At the center of this transformation is a selective catalyst that works under heat and pressure to convert the carbon monoxide and hydrogen into larger, more useful compounds. Currently the FT reaction is successfully used for fuel production from coal (CtL = Coal-to-Liquid) or natural gas (GtL = Gas-to-Liquid). Variations on this synthesis pathway soon followed for the selective production of methanol, mixed alcohols, and isosynthesis products. Another outcome of Fischer-Tropsch Synthesis was the hydroformylation of olefins, discovered in 1938 [53]. Catalysts play a pivotal role in syngas conversion reactions. In fact, fuels and chemicals synthesis from syngas does not occur in the absence of appropriate catalysts [53].

The formation of tars, and measures to deal with their removal, are significant challenges in biomass gasification. Advances in catalyst preparations are also needed in order to make large-scale biomass to liquid facilities practical [52].

Alternatively, syngas can be converted into alcohols, such as ethanol and butanol, or other chemicals, such as organic acids and methane, through syngas fermentation. The main advantages of this microbiological process are the mild process conditions (ambient temperature and pressure), lower sensitivity of the used microorganisms towards sulfur (resulting in reduced gas cleaning costs), independence of the H2:CO ratio for bioconver­sion, aseptic operation of syngas fermentation due to generation of syngas at higher tem­peratures, no issue of noble metal poisoning and a higher reaction specificity. Biological catalysts (such as Clostridium ljungdahlii, Clostridium autoethanogenum, Acetobacterium woodii, Clostridium carboxidivorans and Peptostreptococcus productus) are able to fer­ment syngas into liquid fuel more effectively than the chemical catalysts (e. g. iron, copper or cobalt) [54, 55]. Low volumetric productivity, poor solubility of gaseous substrates in the liquid phase, inhibition of microorganisms, syngas quality and product recovery are the major issues to be addressed in order to make syngas fermentation more economically feasible [16, 56, 57].

There are a variety of technologies for the production of heat and electricity from biomass, including combustion, gasification, pyrolysis, hydrothermal liquefaction, and anaerobic digestion. Additionally, these technologies can be combined in the idea of the integrated biorefinery to produce electricity, heat, biofuels, and bioproducts. The main benefit to using these technologies is the reduction of greenhouse gas emissions, but there are still a number of challenges these technologies must overcome. For example, biomass continues to be high in moisture, low in density, unstable, and produces less energy than coal and natural gas, therefore increasing the cost per unit energy produced. Developing technology improve­ments and innovative uses will help overcome some of these challenges. For example, gasification and anaerobic digestion could be used in small modular systems with internal combustion or generators to provide electricity to remote areas. Small modular system could also be used to fill in gaps with distributed energy generation systems; however, large systems will continue to face issues with insufficient infrastructure, storage of large quantities of biomass requiring large areas, and current supply systems designed for local systems, and environmental regulations being driven by individual states.

There are a number of drivers that are pushing industry to adopt biopower technologies, including the Renewable Fuel Standard and the Renewable Portfolio Standards [30, 31]. These policies give a financial incentive to produce bioenergy. In spite of these policies, the increase in bioenergy production has not accelerated as expected. There are a variety of barriers that need to be addressed for the bioenergy industry to succeed. These include:

• Access to low cost sustainable biomass.

• Additional research into technologies that increase efficiencies and decrease operational costs.

• Additional policies and incentives, such as a CO2 tax.

The bioenergy industry has already proven to have a place in the overall energy portfolio and will continue to grow. The only questions are how fast and in what direction.

Sofie Dobbelaere, Tom Anthonis, and Wim Soetaert

Centre of Expertise for Industrial Biotechnology and Biocatalysis,
Faculty of Bioscience Engineering, Ghent University, Belgium

2.1 Introduction

Until the last century, plant-based resources were largely focused towards food, feed, and fiber production. In addition, biomass has been a major source of energy for mankind worldwide. However, plant/crop-based renewable resources are also a viable alternative to the current dependence on non-renewable, diminishing fossil fuels, to alleviate greenhouse gas (GHG) emissions, and a strategic option to meet the growing need for industrial building blocks and bioenergy. Indeed, biomass seems a very promising resource for substituting fossil hydrocarbons as a renewable source of energy and as a sustainable raw material for various industrial sectors. Over the past decades, the use of biomass has increased rapidly in many parts of the world, mainly to meet the often ambitious targets for energy supply.

Developing biomass into a sustainable, domestic source of affordable biochemicals and biofuels requires the flexibility to use a wide variety of, preferably, non-food biomass resources. Lignocellulosic biomass such as agricultural and forestry residues and herba­ceous energy crops can serve as low-cost renewable feedstock for many, next-generation, bio-derived products. However, the use of biomass as feedstock for the production of materials, products or energy requires new technologies well adapted to the physical cha­racteristics of the biomass. The use of plant/crop resources for energy, or as basic building

Cellulosic Energy Cropping Systems, First Edition. Edited by Douglas L. Karlen. © 2014 John Wiley & Sons, Ltd. Published 2014 by John Wiley & Sons, Ltd.

blocks for industrial production, has been limited because of a poor fit with the hydro­carbon processing system that has been successfully developed to utilize fossil fuels [1]. Although biomass is a nearly universal feedstock, characterized by a high versatility, domes­tic availability, and renewability, at the same time it has also its limitations. Over the years, numerous research and development efforts have been undertaken to develop and apply new cost-efficient conversion processes for lignocellulosic biomass. This chapter gives an overview of the conversion technologies for liquid fuels and biochemicals.