在后勤方面的限制开发-专用于美国东南部的大型生物能源系统

发布时间：2016-05-17 06:24

Abstract: 摘要

多生物质能源原料资源已经提出了许多或所有这些可能最终需要。本文着重对多年生木质纤维原料。在美国，perlack和同事们估计，2050 13亿毫克的量能够可持续地收获每年。然而，超过一半的草本生物量在他们的估计来自作物残茬，一个来源，提出了大量的关注。土地资源的可用性和经济发展前景的土地，从粮食生产的能源生产的影响，如增加的人口和更高的生活水平，如未知的变量。目前正在给玉米和其他淀粉或谷物作物，可以很容易地转换为乙醇的努力。虽然这些作物已启动燃料乙醇产业，他们不太可能满足日益增长的需求，对环境的负面影响更大的潜力，以及它们的使用将“食品与燃料”的争论。2007能源法案规定的燃料乙醇年生产1360亿升，等。从可再生原料的2022，这一目标将极大地影响美国农业。2009、在可再生燃料生产的增加是来自非淀粉的来源，但生产“纤维素乙醇”目前由变频技术有限公司。除了转换的限制，所有的生物能源技术需要的设备系统，能有效地收集，存储，和提供庞大的分布式生物质的能源植物。Multiple biomass-for-energy feedstock resources have been proposed and many or all of these may ultimately be needed. This paper focuses on perennial lignocellulosic feedstocks. In the United States, Perlack and colleagues estimated that by 2050 1.3 billion Mg of biomass could be sustainably harvested annually. However, over half of the herbaceous biomass in their estimate was derived from crop residues, a source that presents numerous concerns. Availability of land resources and the economic prospects for diverting land from food to energy production is further clouded by unknown variables such as the impact of increased human populations and higher living standards. Much effort is currently being given to corn and other starch or grain crops that can be readily converted to ethanol. Although these crops have served to jumpstart the fuel ethanol industry, they have much less potential to meet the growing demand, much greater potential for negative environmental impacts, and their use feeds the “food versus fuel” debate. The 2007 Energy Bill mandated annual production of 136 billion L of fuel  ethanol, etc.. from renewable feedstocks by 2022, a goal that will greatly impact U.S. agriculture. From 2009, all increases in renewable fuel production are to come from nonstarch sources, yet production of “cellulosic ethanol” is currently limited by the conversion technologies. In addition to conversion constraints, all bioenergy technologies will require equipment systems that can cost effectively collect, store, and deliver bulky distributed biomass to the bioenergy plant. These systems will emulate commercial systems that move herbaceous crops  e.g., cotton and sugarcane. to processing plants; however they must be fine tuned to address the requirements of the crop and the constraints of the land base. Important interactions occur between each component of the supply chain—agronomy, logistics, and processing—that are best not studied in isolation. Significant social issues also stand to influence landowner decisions regarding market entry and these will affect the function and profitability of a bioenergy plant. The business plan must provide a “win-win” for both the feedstock supplier and the plant owner. Other large questions about the development of bioenergy resources reside outside the system, e.g., bioenergy systems may be more cost competitive if the policy allows them to benefit from the potential ecosystem services they provide such as sequestering carbon. DOI: 10.1061/ ASCE EE.1943-7870.0000123 CE Database subject headings: Biomass; Ecology; Economic factors; Logistics; United States. 

Introduction 介绍

The 2007 Energy Bill  2007 Energy Independence and Security Act. mandates the annual production of 136 billion L of fuel ethanol from renewable feedstocks in the United States. Current U.S. annual fuel ethanol production is about 25 billion L, with more than 90% derived from corn. Over the past 3 years, as the fuel ethanol industry has increased to over 120 plants, corn prices have increased in the United States, consequently food prices have increased. The USDA estimates that food prices will increase at about 4% in 2008, which compares to a recent historical trend of 2–2.5%. There is a concern in the polity that diversion of 1Professor, Biological Systems Engineering, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061  corresponding author . 2Associate Professor, Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061. 3Professor, Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061. 4Professor, Agricultural and Applied Economics, Virginia Polytechnic Institute and State Univ., Blacksburg, VA 24061. Note. This manuscript was submitted on June 16, 2008; approved on July 14, 2009; published online on July 16, 2009. Discussion period open until April 1, 2010; separate discussions must be submitted for individual papers. This paper is part of the Journal of Environmental Engineering, Vol. 135, No. 11, November 1, 2009. .ASCE, ISSN 0733-9372/2009/111086–1096/$25.00. corn to produce fuel ethanol has increased food prices. The goal set by the 2007 Energy Bill has put direct pressure on the bioenergy industry to commercially produce fuel ethanol from lignocellulosic crops. Nobel laureate Dr. Richard Smalley stated: “Solving the energy problem will solve a number of other problems facing mankind.” Numerous national leaders, with varied backgrounds and perspectives, have said that, if major breakthroughs in bioenergy are to be achieved in the early 21st century, research and interest in bioenergy must be given the same national priority as placing a man on the moon in the 1960’s. Of great importance at that time was the increase in interest in engineering that resulted from this national goal. Equivalent commitment is needed to “engineer the bioenergy economy.” 

Scope 范围

Bioenergy, energy derived from biomass, offers promise as a renewable energy source because it potentially can be produced to meet a continuous demand and potential forms can be used with existing infrastructure. Other renewable energy sources such as solar and wind are also certain to be part of the future energy mix. However, without expensive storage technologies, solar and wind will always be intermittent energysources and their use will require some remolding of the nation’s infrastructure. Despite the 1086 / JOURNAL OF ENVIRONMENTAL ENGINEERING . ASCE / NOVEMBER 2009  
advantages of fungibility, integrating bioenergy technologies into our energy network presents unique engineering challenges. The most common biomass conversion technologies associated with the term “bioenergy” are:  1. direct combustion to produce heat  electricity and process steam ;  2. grain fermentation to produce ethanol  or “grain ethanol” ;  3. extraction of plant or animal oils to produce a liquid fuel  often referred to as “biodiesel” ;  4. degradation of plant fiber into simple sugars for fermentation  referred to as “cellulosic ethanol” ; and  5. thermo-chemical conversion of biomass to produce a pyrolitic oil  referred to as “bio-oil” . The front end of all bioenergy plants is the same: biomass is produced on land surrounding the plant and is harvested, stored, and delivered to meet a given demand schedule. Plants that can operate continuously have a competitive advantage over plants that can operate only part of the year. Thus, most business plans currently in development are based on year-round operation. We have written earlier about some broader challenges to developing large-scale bioenergy production systems  Fike et al. 2007 . This paper focuses on logistic issues associated with the harvest, storage, and delivery of biomass to a bioenergy facility. Since the woody biomass industries are well established, we will focus on the emerging herbaceous biomass industries 

Current Herbaceous Biomass Industries: Seed Cotton and Sugarcane 目前的草本生物量产业：种子棉花和甘蔗

The operation of existing agricultural systems that use herbaceous biomass as a feedstock can illustrate important harvest-storagelogistics principles. It is logical to expect the expanding bioenergy industry to use these principles and improve upon them where possible. Cotton  Gossypium spp.. harvesters collect the fiber and seed and dump the seed cotton into in-field hauling wagons that, in turn, side dump into a module builder parked at the edge of the field. These machines at the field’s edge then compact the seed cotton, forming 7-Mg blocks known as modules. Each module is hauled by a truck equipped with a special body that allows the bed to tilt until auxiliary wheels contact the ground. These wheels then pull the truck back under the module. Load time is typically about 5 min. Upon arrival at the cotton gin, the truck driver can set the module on the ground in the storage yard or place it on a module conveyor that feeds the seed cotton directly into the gin. Unload time is typically less than 5 min. Material flow from at-gin storage is achieved with a module hauler that can pick up and move modules from storage to the conveyor as needed to maintain a continuous flow of seed cotton into the gin.When comparing a bioenergy industry to cotton ginning, it is important to note three differences:  1. the duration of the ginning season is short   3 months. while bioenergy processing presumably will occur continuously throughout the year;  2. the farmgate value of seed cotton is $0.77/kg versus $0.07 to 0.08/kg for herbaceous biomass; and  3. a cotton gin is a mechanical plant that can be stopped and started much more readily than a bioenergy plant. Several principles from the cotton industry should be considered by a bioenergy industry dependent upon herbaceous biomass feedstocks: 1. Rapid loading and unloading is valued to maximize truck productivity. Module haulers are uniquely designed trucks that are not used for other purposes; thus they are idle 75% of the year. However, such trucks are loaded and unloaded in 5 min, which is a rate of productivity that is not exceeded by any other biomass industry except the grain industry. 2. Hauling is provided by the gin operator. Building the module is the farmer’s responsibility and each farmer has to own, or contract for, a module builder. 3. Easy flow of material into and out of at-gin storage is a high priority. The gin can operate continuously if there is a module hauler on site to continuously supply modules from at-gin storage. In the sugar industry in South Florida, the harvest season is 4.5 months and the sugarcane  Saccharum sp.. is processed as it is harvested. Deterioration is so rapid that no long-term storage is possible. In the field, cane harvesters cut the cane stalks into billets approximately 30-cm long and deposit these billets into side-dump wagons. These billets are dumped from the wagons onto a conveyor at a field loading site and the conveyor loads the cane into two bins on specially designed tractor-trailer trucks. The trailers are equipped such that the bins can be side dumped onto a conveyor that feeds the sugar mill. If the material is not needed immediately when the truck arrives, the bins are removed with a forklift and stacked two-high in a short-term storage yard. A sugar mill operates continuously; thus, at-mill storage is needed to supply material for night operation. The sugar industry in South Florida is an example of plantation agriculture. The sugar company owns the surrounding production fields, the roads to those fields, the harvesting equipment, and the hauling equipment, as well as the mill. This arrangement allows the industry to operate 7 days/week with just-in-time delivery of feedstock because all operations are under central management. Truck productivity, defined as achieved loads per day divided by theoretical loads per day, can reach 70% on some days. It is unlikely this productivity can be achieved at a bioenergy plant where the trucks travel on public roads. Because of its integrated structure, the South Florida sugar industry provides a “best case” reference point for comparison with any herbaceous biomass logistics system. 

Biomass Production Potential and Feedstocks 生物质生产潜力和原料

An “energy crop,” as we shall use the phrase, is any crop grown for the express purpose of harvesting its biomass to produce bioenergy. It is valid to consider bioenergy systems that employ biomass derived from wastes and residues  e.g., corn stover and wheat straw. and this will be done later to compare the potential of these sources. 

Biomass Supplies 生物质供应

The DOE and the USDA released a joint report on the country’s potential to sustainably produce biomass supplies sufficient to displace 30% of its present petroleum consumption  Perlack et al. 2005 . The salient feature of the DOE/USDA report was the estimate that the existing land base could both supply that quantity of biomass  1.3 billion Mg. and simultaneously meet existing food, feed, fiber, and export demands. The report stated that this could be achieved by mid-21st century with only modest changes in land use and production practices and that agricultural land would supply the bulk of these biomass resources. The 1.3-billion-Mg goal assumes significant gains in plant productivity and nutrient use efficiency—along with greater soil and water conservation. Without such progress in efficiency and conservation, annual cropping systems—already hugely vested in fossil fuel inputs—are unlikely to meet the desired end of displac-JOURNAL OF ENVIRONMENTAL ENGINEERING . ASCE / NOVEMBER 2009 / 1087  
ing significant fossil energy resources and mitigating impacts of fossil fuel consumption. Although biotechnological gains may indeed help achieve the desired advances in production and efficiency  e.g., see Ragauskas et al.  2006  , such promises are not guaranteed. There will also be concerns regarding the law of unintended consequences as it relates to genetically engineered plants. Even if benign, recent experience suggests transgenic plant materials may face high hurdles before their release. 

Food versus Fuel 食品与燃料

Potential competition between land uses—particularly vehicular fuel versus human fuel, i.e., food—is a concern frequently voiced about bioenergy systems  e.g., see Brown  2006  . In terms of plant biomass, more than 30% of global net primary production is already appropriated for human use  Vitousek et al. 1986; Rojstaczer et al. 2001 . How such land use changes might affect food costs has also been modeled in the context of a carbon tax strategy to reduce greenhouse gas emissions  Daniel et al. 2007 . With increased carbon taxes, large increases in food prices have been predicted, as the taxes would support replacement of food crops with energy crops. Several of these skeptics’ assumptions might be questioned. For example: . Will bioenergy systems only be suitably productive on the most fertile soils? . Would policy makers allow a rapid doubling of food costs? . Will the U.S. government make carbon tax or cap-and-trade systems a reality? Whether energy crops can both compete and cooperate with existing and future production systems and meet societal needs  e.g., see Giampietro and Ulgiati  2005 . are also large and looming questions. The host of interactions among the numerous variables that may affect dedicated bioenergy systems create uncertainties about the structure and function of large-scale biomass-to-energy schemes  Hoogwijk et al. 2003 . For example, the dynamics of population growth, increased standards of living and more energy-intensive diets, potential changes in food production systems, increases in plant productivity, changes in land use, environmental impacts, and social consequences are among the numerous interacting factors affecting the feasibility, function, environmental impact, and economic return of bioenergy systems  Fritsche 2004; Hoogwijk et al. 2003 . In 2004,  6% of the energy consumed in the United States   100EJ  100. 102 J . was supplied by renewable sources, with 47% of those supplies generated from biomass  EIA 2007 . For this accounting, the constituents of biomass included black liquor, wood and wood wastes, municipal wastes, landfill gas, sludges, and even old tires—in addition to ethanol from corn  Zea mays. and other agricultural crops and by-products. Clearly, with such a small percentage of the current U.S. energy market, bioenergy, specifically herbaceous biomass, has a long road ahead to meet substantial portions of energy demand. A vast array of feedstock sources has been, or is being, explored in the United States and globally. We acknowledge the importance of using woody crops and waste streams as feedstock for various energy products. The quantity of energy needed is so great that all options must be explored. However, the focus of this paper is on an industry based on dedicated herbaceous energy crop  HEC. production and we now turn our attention to that option. 

Herbaceous Bioenergy Systems—Biological Considerations/Constraints in Sustainable Development and Design 草本能源系统生物因素/约束的可持续发展设计

Although forested lands by far make up the greatest potential in terms of land-based inventory, agricultural lands have greater potential to supply biomass feedstocks given their much greater management intensity  Perlack et al. 2005 . Cool-season cereal grains such as wheat  Triticum aestivum. and barley  Hordeum vulgare , primarily explored by European researchers  e.g., Loyce et al.  2002 ; Rosenberger  2005 ; Rosenberger et al.  2002  , are garnering increasing interest in the United States. Sorghums  Sorghum sp.. are being evaluated for grain quality  e.g., Corredor et al.  2006 . as well as for sugar and biomass production. Among oilseed crops, soybean  Glycine max. for biodiesel has received the most attention in the United States. Canola  Brassica rapa. and sunflower  Helianthus sp.. are also of interest for their oils and sugar beet  Beta vulgaris. may serve the ethanol market also; but investigation of these crops for biofuel purposes has primarily been conducted in Europe  e.g., Bauen  2001 ; Jaggard  2005 ; Powlson et al.  2005 ; Rosa  2006 ; Schweitzer  2006 ; Usta et al.  2005 ; Zeddies  2004  . Despite the number of potential feedstocks investigated—and our list is by no means exhaustive—corn  maize. takes center stage in the current U.S. bioenergy market, where emphasis is being given to ethanol production. Indeed, ethanol from corn had a 99% market share and 18% annual growth rates over a 5-year period as of 2005  Scaff and Reca 2005 . Impressive as these numbers may seem, even if all the corn  and the soybeans. in the United States could be harnessed for liquid biofuels production, this would only meet about 12% of U.S. energy needs  Hill et al. 2006 . Moreover, there are a number of attendant problems with annual cropping systems. While these food and feed crops may have some potential for jump starting an ethanol industry, we argue that they are not suitable feedstocks for long-term sustainable HEC systems  Fike et al. 2007 . By definition, a sustainable HEC system must produce biomass feedstocks in quantities that provide economic benefit while creating minimal or only positive environmental impacts ad infinitum. Setting aside the important and not fully resolved issues of whether producing ethanol from corn results in positive fossil fuel offsets and atmospheric carbon mitigation  Farrell et al. 2006; Hill et al. 2006; Patzek 2004; Patzek and Pimentel 2005; Sartori et al. 2006; Spatari et al. 2005 , corn production per se poses significant environmental risks, raising the question of whether its long-term use in bioenergy systems can be justified. Inherent risks of corn production include the potential for high rates of soil erosion and degradation and downstream effects of nutrient runoff. We argue  Fike et al. 2007 , as has been argued by others  Brown et al. 2000; DOE 2006; Kort et al. 1998; NRCS 2006; Ragauskas et al. 2006 , that perennial species  whether herbaceous or woody. are more likely than annuals to offer the consistent yields, the feedstock qualities, the soil protection/ amelioration potential, and the lowered inputs that would make their production both economically viable and environmentally benign.  The 1.3-billion-Mg-per-annum scenario offered by DOE/ USDA  Perlack et al. 2005. relies on perennials for less than 40% of the biomass tonnage to be gleaned from agriculture.. Indeed, perennial crops might even improve degraded lands by increasing soil organic matter and improving other chemical and physical 1088 / JOURNAL OF ENVIRONMENTAL ENGINEERING . ASCE / NOVEMBER 2009  

Fig.1.Breakdown of total delivered cost of feedstock into three cost categories to organize discussion properties  Blanco-Canqui and Lemus 2005; Bransby et al. 1998; Frank et al. 2004; Lemus and Lal 2005; McLaughlin and Kszos 2005; Sartori et al. 2006 . So, which perennial herbaceous species will be the one to ignite the bioenergy industry? This is a “trick” question because it is unlikely that any single species will provide “the” answer. Different climatic zones will best suit  or will be best suited by. different energy-crop species. Furthermore, it may be difficult to identify today those species that will become the feedstocks for future bioenergy plants because they may not yet even be on the drawing board. DOE  2006 suggested that the species that will serve as feedstocks in the 21st century will likely be highly engineered from existing species—species that may not yet have been identified. There has been much talk of switchgrass  Panicum virgatum. as an energy crop  McLaughlin and Kszos 2005; Parrish and Fike 2005; Parrish et al. 2008 and Miscanthus  Miscanthus x giganteus is becoming a household word in bioenergy circles as well  Lewandowski et al. 2000, 2003 . However, no HEC species will figure prominently in the new energy industry if it does not lend itself readily to the genetic manipulation  by conventional breeding initially but increasingly by bioengineering that will be required to optimize yields and feedstock utility for bioenergy  Fike et al. 2007 . 

Logistic Challenges for Bioenergy Industry 对生物能源行业的物流挑战

Development Most considerations of engineering issues for bioenergy systems dwell on the end process, however, logistic constraints attendant to harvesting, handling, storing, and transporting low-density feedstocks must be addressed if we are to create practical and commercially viable systems. To create an economy of a size sufficient to support a bioenergy plant at commercial scale will require efficient mechanized systems for moving feedstock from the field to the plant. Indeed, without such systems, the cost of low-density feedstocks at the factory gate could double the price at the farmgate. Before discussing logistic issues, it is appropriate to review the sources of costs for feedstock delivered to a bioenergy plant. These costs can be broken into three main categories  Fig. 1 . Costs at the farmgate include all production inputs  land preparation, seeding, fertilizer application, and pesticide application. plus the harvest operations  mower conditioning, raking, baling, in-field hauling, and storage . The load/haul and the receiving facility categories include all costs from the farmgate to the entry of a continuous stream of material into the bioenergy plant. All three cost categories are linked and have important interactions; thus we are not suggesting they can be separated other than for discussion purposes. It is not appropriate to give specific cost estimates, however, some context can be gained by considering that the farmgate category may be as high as 70% of the total delivered cost with the remaining 30% being the combined load-haul and receiving facility costs. The vertical line in Fig. 1 delineates a division between “agricultural” operations and “industrial” operations. The delineation is located at the satellite storage location  “SSL”  Cundiff et al. 2004 . For purposes of discussion, it is appropriate to think of this as a storage site within 3.5 km of the production field. Greater distances increase the cycle time of the in-field hauling equipment, thus reducing its hauling capacity  Mg/hr . Conversely, smaller distances decrease the amount of biomass that can be accumulated at a given SSL, which disadvantages the hauling contractor and increases their hauling costs. At-plant storage of a year’s supply of feedstock is not being considered in any business plan known to these writers. The quantities required for year-round operation are just too vast for a central storage, plus the cost to flow the material into and out of such a large storage is prohibitive. Woody biomass is “stored on the stump” in the Southeast and harvested year-round. The best commercial example for herbaceous feedstock systems is the Abengoa Bioenergy plant being built in Kansas. This plant uses a series of satellite depot locations  SDLs to receive large square bales of corn stover from production fields. These bales are stored in the SDL for year-round delivery at the required number of truckloads each week. The SDL concept is identical in function to the SSL concept but it is implemented differently. Abengoa purchases the corn stover in the field after the grain harvest and is responsible for harvesting, in-field hauling, storage, and highway hauling to the plant. With the SSL concept, the farmgate contract calls for the farmer to harvest the feedstock and place in the SSL. The bioenergy plant then takes possession and assumes responsibility for highway hauling, a procedure that insures the needed uniform year-round delivery. Asking 350 farmers to deliver feedstock “when they have time” is not a workable plan. 

Feedstock Resources, Harvest Technologies, and Downstream 原料资源，，收获技术，下游

Effects Feedstock resources and management will have important implications for all subsequent logistic operations. While much has been made of the potential availability of harvest residues such as corn stover as a feedstock  Perlack et al. 2005 , it is not clear the degree to which logistic requirements of these systems were considered in the assessment. Biomass resources such as corn stover—and other harvest residues that can only be collected within a narrow window in time—will be constrained by weather conditions at harvest. Although this window may be widened by the development of “one-pass” harvest technologies  Atchison and Hettenhaus 2004 , these time frames are still compressed relative to other feedstock systems and the result is that greater storage capacity at the SSL is required. Currently, for example, corn stover in the upper Midwest is collected during 5 weeks in the fall after the grain is harvested and before the fields are covered with snow. In contrast, using delayed harvest techniques  meaning that the biomass is allowed to dry standing in the field , feedstocks such as switchgrass can be harvested over a 6-month period in the Southeast. HEC fields surrounding an SSL may be managed such that the JOURNAL OF ENVIRONMENTAL ENGINEERING . ASCE / NOVEMBER 2009 / 1089  
SSL can be filled more than once per year. While this may reduce required storage capacity and costs, there will be trade-offs within other links of the delivery chain. For example, if a feedstock such as switchgrass is harvested both in summer and winter, the summer harvest must necessarily be dried in the field—subjecting it to the vagaries of weather—or it must be hauled wet, increasing transport costs. There may also be process implications as the wet feedstock can deteriorate rapidly. Typically, a material harvested wet will be delivered directly to the utilization point. The best example of this is found in the sugarcane industry which utilizes bagasse—the process residues after soluble sugars are extracted from the cane. Bagasse can be burned to produce electricity and process steam for the mill and the sugar industry has developed wet-pile storage methods to preserve this material for use after the harvest season. This technology may also be applicable to corn stover  Atchison and Hettenhaus 2004; Kumar et al. 2004, 2005 . The key difference between a processing residue, like bagasse, and a harvest residue, such as corn stover, is the transport cost. Bagasse is essentially a captured resource: it has already been collected and delivered to a central point. Materials such as corn stover, however, must be collected and hauled in from surrounding fields. Hauling a 20-Mg load of 40% moisture stover means that the truck is hauling 8-Mg water. The cost to operate the truck is the same as for a 20-Mg load of hay at 15% moisture content but the truck is hauling only 3 Mg of water—thus achieving a 42% increase in dry matter delivered. While alternative transport systems such as rail or pipelines for wet harvest residues have been envisioned  Atchison and Hettenhaus 2004; Kumar et al. 2004, 2005 , new infrastructure will be needed to bring these to reality. Along with feedstock sources, the harvest method  and its impact on the system. bears some consideration. Harvest costs are well defined by mature technologies, both for perennial grass crops  Cundiff 1996; Cundiff and Marsh 1996. as well as for crop residues Hoskinson et al. 2006; Kumar and Sokhansanj 2006; Sokhansanj et al. 2002. and grain crops. In grain production systems, current research is being devoted to modifying harvest systems that can both capture and separately collect the grain and crop residues  straw or stover . Early work on one-pass harvest systems for corn was performed by Peart  1980. and more recent efforts have been reported by Quick  2002. and Shinners et al.  2005, 2007. among others. Such systems also are of interest for small grain crops  e.g., Hoskinson and Hess  2004  . While one-pass harvest systems have promise, they also raise several issues. Harvest time is typically greater than occurs when only the grain crop is collected and component recovery can be variable  Shinners et al. 2007 .In addition, in-field hauling may be problematic because it can increase the labor and equipment demands on the producer, who may already be challenged to get the grain crop out of the field. Collecting the crop residues in a trailer following the combine is another option  e.g., see Hoskinson and Hess  2004 . but the material still must be hauled or dumped for later handling. One potential adoptive improvement over the latter would be to pull a large square baler behind a combine to bale the residue.  The writers are aware that this has been done successfully by Australian farmers.. Such harvesting technology has potential in extensive cropping systems where the fields are large and the conditions are dry during the harvest season but they will not be practical in high-rainfall areas and areas with small irregular-shaped fields on rolling terrain. Dedicated biomass systems based on perennial grasses have several options for harvest and handling, including chopping with direct haul, chopping with module handling, or cutting and baling. Chopping provides size reduction and converts the biomass into a flowable material with certain handling advantages. However, chopper-based systems that rely on direct haul are disadvantaged because a chopper cannot move in the field unless some form of collection unit  e.g., a dump wagon or truck. is in place to receive the harvested material. Coupling the harvest and in-field hauling operations is a disadvantage. Adopting cotton module technology to dedicated biofuel cropping systems may be one way in which in-field chopping can work  Popp and Hogan 2007 . Because the material is already chopped, module-based handling systems may present an advantage at the process facility. However, the greater requirement for labor and for storage  tarps in this case. would be limitations for the chopping/module system and the economic efficiencies for each of these systems remain to be determined  Popp and Hogan 2007 . Moisture content is also an issue. If high-yielding tropical grasses are harvested at a higher moisture content, i.e., greater than 30%, then the modules will probably needto be encased in plastic to limit oxygen and thus promote ensiling. Also, experiments are needed to determine if modules of chopped grass will hold together as well as modules of seed cotton. Such logistic considerations have the potential to present challenges for adapting high-yielding tropical grasses to bioenergy systems. Whatever species is chosen must readily meet logistical constraints and those that can only be harvested by chopping may face competitive disadvantages. Among currently available harvesting technologies, baling may have the greatest potential because it provides an interruption between the harvest and in-field hauling operations. For low-rainfall areas of the country, the big square bale is a good choice  Wright et al. 2006; Mukunda et al. 2006 ; but round bales are better for aggregating bioenergy feedstocks in humid regions. The outer layer of a round bale made from coarse grasses acts as a thatch layer, protecting the bale from rain penetration and minimizing losses when stacked in a single layer and held at ambient storage conditions  J.S. Cundiff, unpublished data . This logistical issue has economic impact: it costs  $7 to 16/dry Mg to store big square bales in a building  Mike Duffy, IA State University, personal communication, March 2007. as compared to $3/dry Mg to store round bales in single-layer ambient storage on a crushed rock surface  Cundiff and Marsh 1996 . Clearly, the round bale has the advantage in the farmgate cost category but round bale systems must also address issues in the load/haul and receiving facility categories. 

Load/Haul 

Biomass is a distributed resource; it must be collected from the surrounding land base and delivered to a biorefinery. In most of the United States, excepting one sugar mill—which uses a railroad network through the sugarcane fields—the required hauling is done with trucks. With over 250,000 trucking companies in the United States  ATM 2004 , this is a mature industry; the costs for all types of hauling, short haul and long haul, are well defined. Costs for rail and truck systems are known  escalating fuel costs aside . Thus, any logistics system based on these modes of transport should be evaluated for its potential to reduce shipping costs. If, for example, a truck-loading technology is developed that costs $4/dry Mg but the reduction in trucking costs achieved by reduced loading time is only $3/dry Mg then the technology is not competitive. 1090 / JOURNAL OF ENVIRONMENTAL ENGINEERING . ASCE / NOVEMBER 2009  
Infrastructure will also be an issue for road-based haul systems. Kumar et al.  2003. calculated that a 900 MW power plant using wood chips would require delivery of 4.3 million dry t/year of fuel. Such a facility would need a 32-Mg load of chips delivered every 4 min. As the writers noted, “it is difficult to imagine a community or a local road system that could accept this traffic density  p. 48 .” 

Preprocess Technologies to Reduce Shipping Costs 

One way to reduce hauling or shipping costs is to densify the material, e.g., by pelleting or cubing. Both of these technologies significantly increase the bulk density of biomass as compared to hay bales but they require careful control of moisture content and have a relatively high energy input to make the pellets or cubes. Such operations will be more competitive if they are done in a stationary setting where electric power, rather than diesel engine power, can be used and if they can operate from one location for as many weeks as possible per year, not just during the harvest season. Densification technologies may have a role in the supply of a very large bioenergy plant, one requiring 2,000 dry Mg/day or more, which would, most probably, have to be supplied by rail-cars. Preprocessing plants could be located along rail lines where trucks could deliver bales to the preprocessing plants for size reduction, densification, and subsequent loading. The railcars could in turn become a mobile storage for biomass, just as unit trains of grain cars serve as mobile storage for large grain mills. In addition to optimizing transportation  by limiting truck hauling distance , such a multifacility preprocessing plant system would have the capacitance needed to adjust for times when one of the preprocessing plants was “down” for maintenance or other factors. 

Receiving Facility 

The receiving facility coordinates all activities at a bioenergy plant associated with the flow of material into and out of at-plant storage to achieve a continuous flow of feedstock. Bioenergy plants will operate like chemical plants in that the penalty for shutdown will be high. Thus, any logistics system must provide for an efficient flow of material into and out of at-plant storage. Most bioenergy plants being planned today list two objectives for their feedstock supply system  M. Hladic, IOGEN, personal communication, February 2007 : 1. Distributed storage: It is not feasible for plants to operate a central storage facility with enough capacity for year-round operation. The cost of moving material into and out of this at-plant storage is prohibitive, plus there is potential for spontaneous combustion in piles of biomass  Atchison and Hettenhaus 2004 . This constraint is one reason the SSL concept is included in our discussion. The SSL system provides optimum division between agricultural and industrial operations, along with the desired distributed storage. 2. Around-the-clock hauling: Ideally, a biorefinery would have just-in-time delivery of feedstock, meaning that trucks will arrive each hour, 24 h/day, 7 days/week. This goal may or may not be achievable with a practical system; it certainly presents an engineering challenge. However, any logistics plan that provides for 24 h use of trucks will have a lower per-megagram hauling cost and thus a competitive advantage. Fig.2.

The relationship between hauling cost and plant size for production areas having different percentages of total land area planted for feedstock production 

Siting a Bioenergy Plant: Role of Hauling Cost and the Influence of Crop Yield

Economies of scale dictate that a bioenergy plant must be a certain size to achieve a unit processing cost within the desired range. Because feedstock transport costs increase as the size of the production area  the area within a given radius of the plant. increases, there are two key issues for siting a bioenergy plant at a given location: 1. Percentage of land area attracted into production and 2. Yield per unit land area. Thus, a community seeking to attract a biorefinery will need to enlist as many landowners as possible within a given radius to make their proposal competitive. To illustrate these two key issues, consider the costs for a bioenergy plant which requires 20 Mg of feedstock per hour  equivalent to a bale per minute with 0.4-Mg bales at 15% moisture . If this plant were built at a location where only about 18% of the surrounding land area were planted to a dedicated energy crop  e.g., switchgrass averaging yields of 11.4 Mg/ha , it could still have a competitive cost advantage over a bioenergy plant that has attracted twice the percentage of surrounding land area to supply stover or other crop residues  with a typical yield of 4.5 Mg/ha. For a “reduced area” plant supplied from an annular area with rmin=4 km and rmax=17 km, the total haul distance for all loads would be 181,175 km. Increasing rmax by only 1 km  i.e., rmax =18 km. would raise the total haul distance for the same size loads to 190,550 km or 5% more than for the reduced area plant.  See Gallagher et al.  2003. for a detailed description of the mathematical calculations needed.. Yield per land area is a strong driver in the distance-hauled constraint. Thus, considering transportation costs alone, an HEC, such as switchgrass, will always have an advantage over crop residues as a bioenergy feedstock because of the higher yield per unit land area. Hauling costs will decrease as more land around a bioenergy plant is dedicated to energy cropping  Fig. 2 . However, while these costs increase with increased plant size for all production area percentages, this response is weaker—the curve is flatter— for the highest percent production area. Large bioenergy plants may have an advantage in lower processing costs per unit of product but when the higher cost of feedstock is added, the total cost of feedstock and processing may be higher than for a JOURNAL OF ENVIRONMENTAL ENGINEERING . ASCE / NOVEMBER 2009 / 1091  
medium-sized plant. Wright and Brown  2007. have suggested that optimum plant size may be in the range of 240–486 million gal. of gasoline equivalent for biorefineries converting biomass to fuels. However, their estimates are based on 11 Mg/ha yields with 60% of the land supplying the facility. At present, few, if any, areas of the country could meet these production requirements. The fact that biomass is a distributed resource and must be collected from the surrounding land area is a key issue in defining an optimum plant size  Jenkins 1997; Kumar et al. 2003 . Fig. 2 illustrates why investors will seek locations where a majority of the landowners within a given radius will sign a feedstock contract. This requirement presents a very interesting socioeconomic challenge for a local community. The infrastructural concerns  roads, railroads, utilities. and environmental issues  waste water treatment, solid waste disposal. will likely be smaller barriers to bioenergy plant start-up than the feedstock production area issue. Earlier, we made the point that interesting interactions occur between the cost categories in Fig. 1. As an example, increased yields actually reduce average hauling costs because the size of the production area required to supply a plant decreases  Aden et al. 2002 . Thus, the hauling distances  and average hauling costs. are decreased. Some Socioeconomic Factors Affecting Siting Bioenergy plants need guaranteed feedstock volume over a multiyear time horizon. This requirement presents a new paradigm for farming communities that have little experience signing long-term contracts and traditionally make year-to-year production and marketing decisions. Part of the reason for these “in-hand” decisions may be the external factor of land tenure. A producer who does not own the production land and cannot secure a long-term lease for it will be constrained from making long-term contracts for supplying feedstock. A community will not attract a bioenergy plant unless feedstock is available and energy crops will not be planted unless a bioenergy plant is present to provide a market. The best “way forward” is a business plan that provides both parties, plant owners and feedstock producers, a way to share in profits when energy prices increase. The old model where farmers produce a bulk commodity and take it to market to see what they can get for it is not workable in a case where the demanders of the product need to ensure long-term continuous feedstock supply. 

Economic, Policy, and Social Challenges for Deploying 

Dedicated Large-Scale Bioenergy Systems in the United States To this point, we have focused primarily on the tangible “nuts and bolts” challenges faced by a biobased energy and chemicals industry. While such internal questions will certainly affect the industry’s development, the set of broader overarching issues— economic, policy, and social—will likely be greater drivers in affecting the industry’s nature and success. Indeed, public policy is viewed as “the single most important strategy to moving toward a bioeconomy”  BTAC 2002, p. 2 . 

Competitiveness with Existing and Alternative Crop/Land Uses 竞争力与现有的和替代作物/土地用途

As increasing percentages of available land are devoted to energy crop production, prices of foods and products that compete with these land uses will increase. For example, Daniel et al.  2007. modeled this competition and suggested that, under their most likely scenario, real food prices could double by 2050 due to worldwide growth in demand for bioenergy. This would represent a stark change from the 1980–2000 era, where demand for foods grew rapidly but overall world price trends were flat. Switchgrass competes less for land for food crops than other feedstocks so a switchgrass-based bioenergy facility is likely to have a smaller impact on food prices. It will also broaden the ability of farmers across a wider area of the United States to participate in bioenergy production. Walsh et al.  2003 , using a multisector model, found that between 9 and 17 million ha of bioenergy crops  switchgrass, hybrid poplar, and willow , annually yielding from 55 to 171 million dry Mg of biomass, potentially could be produced at a profit greater than the existing agricultural uses for the land. Their study also showed that dedicated biomass production would put upward price pressure  4–14%. on crops such as corn and soybeans as farmland switched from food to energy production  Walsh et al. 2003 . Of course, farmgate prices for biomass crops will drive these pressures on grain commodities. Walsh et al.  2003. estimated that at $1.83/GJ  GJ=109 J , about 8 million ha of bioenergy crops could provide greater profit than existing agricultural uses. With a 33% higher price at the farm gate  $2.44/ GJ , this acreage would more than double to 17 million ha. It should also be noted that this estimate assumed offsetting factors of increased prices for traditional crops and the use of high production management on conservation reserve program lands. These lands are currently not in production and represent about 9% of available cropland. Estimates are based on the National Resource Inventory, 2006. studied grain and cellulosic ethanol systems relative to fossil fuels for the southeastern United States. Feedstock supply in response to price was highly variable but large-scale expansion of bioenergy demands was predicted to propel a 4.6-million-ha increase in agricultural acreage by 2010. Large increases in hay acreage  5.3 million ha. and switchgrass production  7.3 million ha. would come primarily at the expense of pasture acreage while corn and soybean acreages would grow only slightly  0.4 and 0.6 million ha, respectively . Even with these increases in farmed acres, demand growth is predicted to stimulate increases in feedstock prices and in prices for commodities that compete for acreage; the price of biomass grows from $31/dry t in 2005 to nearly $50 by 2014. While government payments are expected to decline  by $13 million over the 10-year horizon considered , farm income would grow by $23 million, with $6 million growth forecast for the southern region. Interestingly, farms in the Midwest were expected to benefit most from these changes. A large unknown variable is the impact of large-scale bioenergy production on land prices: no credible estimates currently exist but increased prices of farm outputs are quickly capitalized into land prices  witness the increase in Iowa land prices during the grain boom in the late 1970’s and the corresponding run-up in prices near ethanol plants during 2006–2008 . If overall cropland acreages increase as feedstock production grows then additional pressures of population growth and urbanization will further exacerbate the effects on land prices. Already in Iowa there is an anecdotal evidence that land prices near ethanol plants are increasing. Standard models of economic geography suggest that, as a production facility is sited in an area, land closest to the facility will increase in value  reflecting the lower cost of trans1092 / JOURNAL OF ENVIRONMENTAL ENGINEERING . ASCE / NOVEMBER 2009  
port to the final demand point . Impacts on land prices decline nearly linearly  with the gradient determined by cost of transportation. with travel distance to the site. If shipping costs are borne by the purchaser of the biomass, this distance effect will not emerge. While such analyses help frame the competition between existing and alternative crops and other land uses, they are somewhat artificial because they are predicated on as yet unknown technological advances. Technology gains will be critical to reduce processing costs and to bolster the potential success of HECto-bioenergy systems  DOE 2006; Schubert 2006 .Itisnot surprising then that the existing literature provides little guidance into costs of lignocellulosic biofuels. Most credible studies estimate capital costs for a 50-million-gal./year  MGY. plant at about $200 million, which is three to four times the costs of a similar-sized grain ethanol plant  Schubert 2006 . In addition, the variable costs for lignocellulose-based ethanol are currently estimated as twice those of corn ethanol. However, as with most infant industries, there is ample room for cost reductions and such reductions are likely. Large reductions in the cost of processing grain to ethanol occurred between 1980 and 1998 because of three factors:  1. higher yields of ethanol per unit of corn;  2. lower costs of enzymes required for conversion; and  3. reduced labor from increased production automation  USDA 2006 . 

Role of Coproducts in Bioenergy Economies 

Bioenergy plants that can manufacture a number of products present an alternative means of reducing costs for fuel generation  Arato et al. 2005 . In such cases, the production of high-valued chemicals and chemical feedstocks may allow for smaller-scale refining facilities—reducing capital requirements and risks—and these products may have more value than the fuel coproduct itself. For instance, the National Renewable Energy Laboratory is partnering with DuPont, other private sector actors, and academic partners to explore potential biobased chemistries  NREL 2003 . An interesting interaction currently exists in the competition of end uses for grain and coproduct effects on grain prices. This present-day low-tech example involves corn, which is being diverted from animal feeds to produce ethanol. The increased demand for grain for ethanol concerns animal production industries reliant on corn-based diets because of the impact on feed costs  Arnot and Gauldin 2007. but the ability to use ethanol coproducts in animal diets can partially offset these price pressures. For example, distillers dry grains with solubles, the primary coproduct from dry-mill ethanol plants, can make up as much as 25% of a dairy feed ration  dry matter basis. and may be suitable for swine and poultry production at 5–15% of the diet  USDA 2006 , lowering dietary protein costs. We are now on the edge of a dramatic improvement in new processing technologies which will make bioenergy production more economically attractive. A major feature of anticipated thermochemical processes is that they can use multiple feedstocks  waste and dedicated crops. which will have a large impact on economic viability and also creates its own logistical issues. 

Regionwide Impacts of Bioenergy Production 

Parcell and Westhoff  2005. summarized a number of studies examining regional impacts of ethanol production, including influence on total economic output, employment, and personal income. The range of impact  dependent on plant size, industry location, and location of feedstock supplies. varied widely  $24– $282 million. in total economic output for the various plant sizes currently in operation. Employment would grow by 3–15 people per million gallons produced and total labor income would grow by around $0.50/gal produced. However, regionwide estimates of ethanol plants tend to overstate their economic impacts  see Swenson 2006 for an excellent overview . Swenson  2006. estimated that a 50-MGY ethanol plant would lead to a $133-million increase in regional value-added production and $25 million in increased income, with about 133 more full-time jobs. The general effects on all markets and prices of using a HEC  switchgrass. to replace petroleum in the United States was examined by McDonald et al.  2006 . Without considering social costs and benefits, switchgrass costs more per unit of energy produced than petroleum and a large-scale switch to cellulosic ethanol was predicted to lower production of other crops while increasing prices ofgrains. The writers suggest that, on net, there will be an economic cost associated with policies to use switch-grass for energy generation under existing technologies. Large-scale production of dedicated energy crops will lead to welfare gains and losses that are distributed unevenly on a global scale. In addition, costs must include changes in relative prices of substitutes and complements in production. However, one important limitation with this kind of analysis is that it does not factor in benefits of reducing pollution, the externalized costs of protecting petroleum supplies, and the impacts of carbon emissions attendant with burning fossil fuels, which we now consider. 

Externalized Costs in Energy Markets and Benefits of Biofuels 

The debate about costs of bioenergy production should not occur without considering external costs associated with production practices. Principle externalities affected by fuel source include air pollution emissions, impacts on the carbon balance, and production-related externalities such as runoff. In a liberalized economy, all actors  producers, consumers, investors, etc.. should face the full costs of their decisions. Energy markets fall well short of this ideal and are dominated by costs that are not reflected in decisions. Some of these costs come from subsidies and other policies  discussed further below. but some are related to the absence of complete property rights in all markets, transaction costs, incomplete enforcement, costs of monitoring, and even market or political power on the part of producers. The cost of climate change from coal-generated electricity has been conservatively estimated at being between

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