JOYCE C. YANG
Department of Energy
O beautiful for spacious skies, for amber waves of grain, for purple mountain majesties, above the fruited plain!
- Katharine Lee Bates (1904)
The bounties of American ingenuity, climate, and soil not only inspire the opening verse of a patriotic song but also establish the United States as the world leader in agriculture (USDA 2013) and forestry1 productivity. Thus it is not surprising that researchers, engineers, industrialists, and policymakers have turned to the nation’s abundant biomass resources to reduce consumption of fossil fuel, be it coal, natural gas, or petroleum. In fact, of all forms of renewable energy consumed in the United States, none rivals that produced from biomass (Figure 1). Furthermore, beyond the total illustrated in Figure 1, a recent report estimates an additional renewable resource of 1 billion dry tons of agricultural residues, woody biomass, and new energy crops that can be sustainably harvested every year (DOE 2011).
Terrestrial biomass feedstocks are typically composed of three major types of polymers: cellulose (homogeneous polymer composed of six-carbon sugars, or C6s), hemicellulose (heterogeneous polymer but predominantly composed of five-carbon sugars, or C5s), and lignin (heterogeneous polymer composed of a
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Bioenergy Technologies and Strategies: A New Frontier Joyce C. Yang Department of Energy O beautiful for spacious skies, for amber waves of grain, for purple mountain majesties, above the fruited plain! – Katharine Lee Bates (1904) The bounties of American ingenuity, climate, and soil not only inspire the opening verse of a patriotic song but also establish the United States as the world leader in agriculture (USDA 2013) and forestry1 productivity. Thus it is not sur- prising that researchers, engineers, industrialists, and policymakers have turned to the nation’s abundant biomass resources to reduce consumption of ossil fuel, f be it coal, natural gas, or petroleum. In fact, of all forms of renewable energy consumed in the United States, none rivals that produced from biomass (Figure 1). Furthermore, beyond the total illustrated in Figure 1, a recent report estimates an additional renewable resource of 1 billion dry tons of agricultural residues, woody biomass, and new energy crops that can be sustainably harvested every year (DOE 2011). Background Terrestrial biomass feedstocks are typically composed of three major types of polymers: cellulose (homogeneous polymer composed of six-carbon sugars, or C6s), hemicellulose (heterogeneous polymer but predominantly composed of five-carbon sugars, or C5s), and lignin (heterogeneous polymer composed of a 1 According to the United Nations Food and Agriculture Organization forest products statistics for 2011, available at www.fao.org/forestry/statistics/80938@180723/en/ (accessed November 25, 2013). 87
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88 FRONTIERS OF ENGINEERING Geothermal, Solar/PV, 0.226 0.158 Wind, 1.168 Biomass, 4.411 Hydroelectric, 3.171 hƩp://www.eia.gov/totalenergy/data/annual/index.cfm#renewable Adapted from Table 10.1 Renewable Energy ProducƟon and ConsumpƟon by Primary Energy Source, 1949-2011 (EIA, AEO 2011) (in Quadrillions of Btu) Biomass Hydroelectric Wind Geothermal Solar/PV Total Year 4.411 3.171 1.168 0.226 0.158 9.135 2011 4.294 2.539 0.923 0.208 0.126 8.090 2010 3.912 2.669 0.721 0.200 0.098 7.600 2009 3.849 2.511 0.546 0.192 0.089 7.186 2008 3.474 2.446 0.341 0.186 0.076 6.523 2007 FIGURE 1 US renewable energy consumption in 2011 by primary energy source, in quadrillion British thermal units (Btus). Total US renewable energy consumed exceeded 9 quadrillion Btus. PV = photovoltaic. Source: EIA 2011. Yang Figure 1_R02544.eps significant component of aromatic molecular units). Biofuels derived from ter- restrial feedstocks are often referred to as “cellulosic” because of their principal biomass component. In contrast, “conventional” biofuels are grain-based (e.g., corn ethanol) and may compete with food and feed markets. Aquatic biomass, such as algae and cyanobacteria, can be a mixture of C5s and C6s polysaccharides along with other classes of biopolymers such as proteins and lipids. Biomass is usually transformed into biofuel through one of two types of pro- cessing: biochemical or thermochemical. In biochemical processing, biomass is typically pretreated with mechanical, chemical, and/or thermal forces to open up the plant cell wall and structure, thus exposing the partially depolymerized mate- rial to microbial enzymes (cellulases and hemicellulases) that attack the chemical bonds to yield monosaccharides. These dilute sugar intermediates are usually fed to a microbe to produce fuels or more refined chemicals.
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BIOENERGY TECHNOLOGIES AND STRATEGIES: A NEW FRONTIER 89 In thermochemical processing the biomass is typically mechanically pre- processed to specific sizes, inorganic contents, and moisture levels, and then subjected to moderate to high pressures and temperatures (with or without cata- lysts) to generate syngas or bio-oil intermediates. These process intermediates are cleaned or stabilized and then exposed to fuel synthesis catalysts to either regenerate the bonds between the C1 units into longer-chained hydrocarbons or hydrocrack larger biomass thermal derivatives to generate fuel blendstocks. The mission of the US Department of Energy (DOE) Bioenergy Technologies Office (BETO) is to transform available domestic biomass resources into fuels, chemicals, and power. It achieves its mission through a diverse and comprehen- sive set of applied research and development (R&D) programs and first-of-a-kind technology demonstrations called integrated biorefineries (IBRs). The BETO strategy is to reduce the risk of biofuel technologies by demonstrating feasibil- ity, process robustness, process control, and scalability to attract private capital for commercialization and market entry. BETO partners are encouraged to use biomass feedstocks that do not compete with food or feed uses, and to develop a suite of versatile conversion technologies that can be deployed in as many regions of the United States as possible to maximize both national and regional benefits. BETO currently focuses on technologies that seek to use cellulosic or algal biomass feedstocks because of more favorable environmental benefits as dem- onstrated by a life cycle analysis of greenhouse gas (GHG) emissions and lower water consumption (Wang et al. 2011; Wu et al. 2009). In fact, to qualify as a cellulosic biofuel for incentives, a 60 percent GHG reduction must be achieved relative to gasoline. Recent Progress The US Department of Energy (DOE) announced the completion of several major R&D programs on cellulosic ethanol at the end of 2012. Achievements on both the biochemical and gasification routes to cellulosic ethanol corresponded with a dramatic reduction in the modeled minimum ethanol selling price from more than $9/gallon, when the program began in 2002, to $2.15/gallon or less in 2012. The many technical performance improvements include better feedstock quality and logistics, pretreatment technologies, more productive cellulolytic enzymes, gas cleanup technologies, and the development of robust microbial and inorganic fuel synthesis catalysts, not to mention a wealth of enabling knowledge gains and breakthroughs contributed by grantees of the DOE Office of Science, National Science Foundation, National Institute of Standards and Technology, and US Department of Agriculture. Concurrent with the R&D achievements that helped reduce key biofuel cost factors, four first-of-a-kind IBRs for cellulosic ethanol were established in the United States and either began producing fuel or will begin to produce it in 2014, and one facility has begun production of cellulosic hydrocarbon fuels (Table 1).
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90 FRONTIERS OF ENGINEERING TABLE 1 Commercial-scale US integrated biorefineries, constructed or under construction, focused on cellulosic biofuels. Start of Target DOE Company construction Feedstock product Process route Location role DuPont 2012Q4 Ag residue Cellulosic Biochemical Nevada, IA R&D ethanol POET- 2012Q1 Ag residue Cellulosic Biochemical Emmetsburg, IA R&D, DSM ethanol IBR Abengoa 2011Q4 Ag residue Cellulosic Biochemical Hugoton, KS IBR ethanol KiOR 2011Q2 Southern Cellulosic Thermo- Columbus, MS None pine gasoline, chemical diesel, and jet INEOS- 2011Q1 MSW, Cellulosic Hybrid Vero Beach, FL R&D, Bio citrus ethanol IBR waste, yard waste, woody biomass NOTE: IBR = integrated biorefinery; MSW = municipal solid waste; Q = quarter. These IBRs represent far more than their technological components: each is the result of successful process integration, scale-up, and construction as well as critical success elements such as feedstock contracts, project management, fuel off-take agreements, seasoned senior management, regulatory clearance, and financing. (Financing these biorefineries has been particularly challenging because the economics are as yet unproven.) Moving Forward Although ethanol can displace the gasoline used for light-duty passenger cars, it cannot be blended with other transportation fuels. One particularly inter- esting variation of the hydrocarbon fuel strategy is to produce an “intermediate” that can be inserted at various processing units in traditional petroleum refineries (Figure 2). The key advantage of this strategy is that several units of operations might be avoided by leveraging existing assets of the petroleum refinery, thus significantly lowering capital costs. There is also a fuel distribution advantage with the biomass-derived blendstock strategy. Accordingly, BETO began in 2010 to shift away from a singular focus on cellulosic ethanol to embrace a more holistic biofuels strategy aimed at replacing an entire barrel of oil by targeting
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Adapting to Refinery Infrastructure: Save on CAPEX Refinery Ready Refinery Ready Biomass Biomass Intermediate A Hydrotreater Intermediate B Reformer Insertion Insertion Crude Point Point Distillation FCC Pipelines Terminals Oil Units Alkylation Insertion Point Coker Biofuel/ Blendstock Adapted from the National Advanced Biofuels Consortium Website FIGURE 2 Proposed insertion points of biomass-derived fuel intermediates into existing petroleum refinery units of operation. Source: Adapted 91 from the National Advanced Biofuels Consortium website, Figure 2_R02544.eps Yang www.nabcprojects.org/biofuels.html; accessed November 25, 2013.
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92 FRONTIERS OF ENGINEERING the production of hydrocarbon, or “drop-in,” fuels that are compatible with the current infrastructure. Stoichiometry presents a major technical challenge for hydrocarbon biofuels: biomass is a relatively oxygen-rich carbon feedstock, whereas hydrocarbons lack oxygen. When the target molecule was ethanol, biomass was an advantaged feedstock compared to petroleum, in terms of basic stoichiometry; but when the target molecule is a longer carbon chain with no oxygen, a biomass feedstock is disadvantaged. This basic chemical balancing act, illustrated in Table 2, will be the key challenge to progress, requiring innovations across the biomass-to-biofuel supply chain. The removal of oxygen in the biomass fuel intermediate will be essential for compatibility with existing crude oil processing streams; but it will also mean a significant loss of yield in the form of either water (requiring a hydrogen source) or carbon monoxide or dioxide (resulting in greater loss of yield from the original biomass). Hydrogen can be derived from natural gas using processes like methane reforming; however, the impact on the GHG reduction for this option should be considered. On the other hand, losses of carbon as CO2 are also unpalatable and negatively impact the GHG profile. At least one partial solution is to diversify the product slate. If hydrocarbon fuels cannot contain oxygen molecules, then it is possible that a marketable coproduct that is “oxygen rich” (defined here as having a carbon:oxygen, or C:O, ratio less than 1) can be made alongside the fuel. It is also likely that such a coproduct could enhance the economics of the overall conversion process. The Department of Energy has identified several such value-added chemicals, includ- ing sorbitol, xylitol, aspartic acid, and diacids (Holladay et al. 2007; Werpy et al. 2004). TABLE 2 Stoichiometric relationship between biomass and petroleum feedstocks and different fuel products. Relative Elemental Fuel Products Feedstocks Distribution by Mass (AFDW Ultimate Analysis) Gasoline Diesel Ethanol Petroleum Biomass C 0.86 0.84 0.52 0.84 0.50 H 0.13 0.13 0.13 0.11 0.06 S 0.01 0.04 0.00 0.05 0.00 N 0.00 0.00 0.00 0.00 0.00 O 0.00 0.00 0.35 0.00 0.44 Total 1.00 1.00 1.00 1.00 1.00 NOTE: AFDW = ash free dry weight; C = carbon; H = hydrogen; S = sulfur; N = nitrogen; O = oxygen. Source: Adapted from Morvay and Gvozdenac 2008.
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BIOENERGY TECHNOLOGIES AND STRATEGIES: A NEW FRONTIER 93 The imbalance in C:O ratio in feedstock and product also requires ever more efficient use of the biomass resource itself. Losses that can occur either under open storage systems (e.g., bale yards or wood laydown yards) to support year- round cellulosic biorefinery operations or from natural disasters (e.g., droughts, flooding) will mean even greater operation expense losses to the hydrocarbon biorefinery versus the ethanol biorefinery. Commoditizing biomass feedstocks can be an effective mitigation strategy; a version of this advanced feedstock concept has been proposed by Idaho National Laboratory (Figure 3) (Hess et al. 2009). A key aspect of commodity-based biomass feedstocks is that different feedstocks can be collected and transported to regionally distributed depots or terminals where they undergo preprocessing and are blended to predefined physio FIGURE 3 Commodity-based biomass feedstock supply logistics system. This design follows the model of the current commodity grain supply system, which manages crop diversity at the point of harvest and/or the storage elevator and thus allows all subsequent feedstock supply system infrastructure to be similar for all biomass resources. Source: Image courtesy of Idaho National Laboratory; Hess et al. 2009.
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94 FRONTIERS OF ENGINEERING chemical specifications and then densified to facilitate storage and handling. Although this method will increase costs because of the additional processing, it is modeled after the existing agricultural grain commodity system, raising the interesting possibility of leveraging the grain distribution network as another infrastructure cost reduction opportunity. This advanced system would also take advantage of a variety of technologies while at the same time continuing to use conventional agriculture and forestry equipment where possible. An intriguing possible alternative is to create an advanced biomass feedstock or feedstock component that changes the overall C:O ratio in vivo to favor hydro- carbon fuel formation. A study published in 2007 suggested that natural plant and microbial oils, such as algal lipids, can be readily converted into hydrocarbon fuels or blendstocks using existing petroleum refinery units (Huber and Corma 2007). Current work on algae suggests that it could soon exceed palm oil (the best terrestrial oilseed crop) (Davis et al. 2012), but the associated cultivation and processing costs result in a fuel product that exceeds $18 per gallon. The relative advantages of using modified biological feedstocks as a means to achieve refinery-ready intermediates versus other approaches will need to be carefully evaluated in terms of both theoretical yields and practical considerations. Conclusion Over the past two decades the United States has consistently pursued strat- egy that involves simultaneously funding research development, demonstrating biofuels technologies, and establishing favorable national policies to incentivize biofuels production to reduce dependence on fossil fuels. Through policies such as the Energy Independence and Security Act of 2007, the Energy Policy Act of 2005, and the Biomass R&D Act of 2000 (Title III), the government has supported innovators across the supply chain, culminating in the first US commercial pro- duction of cellulosic ethanol in 2013. The United States is positioned to benefit not only from an abundance of renewable biomass but also from the use of existing infrastructures across the country. The new frontier of biofuels RD&D will no doubt be full of significant challenges, but scientific and engineering innovators can overcome them by build- ing on a solid foundation of knowledge and leveraging advances already realized in first- and second-generation biofuels. REFERENCES Davis R, Fishman D, Frank ED, Wigmosta MS. 2012. Renewable Diesel from Algal Lipids: An Inte- grated Baseline for Cost, Emissions, and Resource Potential from a Harmonized Model. Golden CO: National Renewable Energy Laboratory. DOE (US Department of Energy). 2011. US Billion-Ton Update: Biomass Supply for a Bioenergy and Bioproducts Industry. Oak Ridge TN: Oak Ridge National Laboratory.
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BIOENERGY TECHNOLOGIES AND STRATEGIES: A NEW FRONTIER 95 EIA (Energy Information Administration). 2011. Annual Energy Outlook. Washington DC: US Depart ent of Energy. m Hess JR, Kenney KL, Ovard L, Searcy EM, Wright CT. 2009. Uniform-format Bioenergy Feedstock Supply System Design Report: Commodity-scale Production of an Infrastructure Compatible Bulk Solid from Herbaceous Lignocellulosic Biomass. Idaho Falls: Idaho National Laboratory. Holladay JE, White JF, Bozell JJ, Johnson D. 2007. Top Value-Added Chemicals from Biomass: II. Results of Screening for Potential Candidates from Biorefinery Lignin. Richland WA: Pacific Northwest National Laboratory. Huber GW, Corma A. 2007. Synergies between bio- and oil refineries for the production of fuels from biomass. Angewandte Chemie International Edition 46(38):7184–7201. Morvay Z, Gvozdenac D. 2008. Applied Industrial Energy and Environmental Management. West Sussex UK: John Wiley & Sons, Ltd. USDA (US Department of Agriculture). 2013. World Agricultural Production. Foreign Agricultural Service, WAP 11-13. Washington DC. Available at www.fas.usda.gov/wap/current/default.asp. Wang MQ, Han J, Haq Z, Tyner WE, Wu M, Elgowainy A. 2011. Energy and greenhouse gas emis- sion effects of corn and cellulosic ethanol with technology improvements and land use changes. Biomass and Bioenergy 35(5):1885–1896. Werpy TA, Holladay JE, White JF. 2004. Top Value Added Chemicals from Biomass: I. Results of Screening for Potential Candidates from Sugars and Synthesis Gas. PNNL-14808. Richland WA: Pacific Northwest National Laboratory. Wu M, Mintz M, Wang M, Arora S. 2009. Water consumption in the production of ethanol and p etroleum gasoline. Environmental Management 44(5):981–997.
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