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a Dep. of Agricultural and Biosystems Engineering, Iowa State Univ., 3202 NSRIC Building, Ames, IA 50011
b Thayer School of Engineering, Dartmouth College, Hanover, NH
c Dep. of Agronomy, Iowa State Univ., Ames, IA 50011
* Corresponding author (rpanex{at}iastate.edu).
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ABSTRACT |
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 TOPNOTES ABSTRACT INTRODUCTIONINTEGRATED CROP-LIVESTOCK...LIGNOCELLULOSIC BIOMASS...NUTRIENT RECOVERY IN BIOMASS...NITROGEN RECOVERY POTENTIAL: AN...NEW BIOENERGY CROPS AND...CONCLUSIONSREFERENCES |
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INTRODUCTION |
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TOPNOTESABSTRACTINTRODUCTIONINTEGRATED CROP-LIVESTOCK...LIGNOCELLULOSIC BIOMASS...NUTRIENT RECOVERY IN BIOMASS...NITROGEN RECOVERY POTENTIAL: AN...NEW BIOENERGY CROPS AND...CONCLUSIONSREFERENCES |
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While there is a potentially large annual flux of agriculturally derived biomass that could be directed toward use as an industrial feedstock for the production of biofuels and other plant-based products, the global agricultural challenge of meeting food and renewable energy needs in the next century is not trivial. It is estimated that yields of the world's three major cereal grains (corn, wheat, and rice [Oryza sativa L.]) will have to increase on the order of 50% per unit land area during the next 50 yr to meet the nutritional demands of an expected 9 to 10 billion people (Evans, 1998; Cassman, 1999). The agricultural community has overcome similar challenges in the past. Cereal crop productivity increased by a factor of two during the later half of the twentieth century, supporting a population doubling that occurred during that same period. In contrast to the challenge of increasing total food production that faced agriculturalists of the post-World War II era, however, the next agricultural revolution will require that governments, producers, and scientists alike place simultaneous emphasis on resource use efficiency and unit area productivity of agricultural systems that are designated to generate both food and energy (Cassman, 1999; Socolow, 1999.). Whereas large production increases during the past century were made possible through the development of improved crop varieties in conjunction with increased fertilizer and pesticide inputs, an expansion of irrigated production, and the conversion of additional lands to agriculture, it is doubtful that a similar strategy will suffice in a future marked by greater constraints on resource availability and environmental concerns. Arable land, petrochemicals and their agricultural derivatives, as well as irrigation water are all likely to become less abundant and increasingly expensive in the coming decades. Continued breeding efforts, particularly those directed toward improving crop stress tolerance, are expected to provide needed productivity gains for the major cereal grains during the next 30 yr (Bruinsma et al., 2003). Nevertheless, if agriculture is to make a significant contribution to global energy security in the twenty-first century, similar breeding advances will also be required for new bioenergy feedstock crops. Moreover, if agriculture is to truly serve as a source of renewable energy, it is absolutely critical that greater attention also be given to the development of feedstock production systems that utilize fossil fuel–derived inputs as efficiently as possible.
The sustainability of an agriculturally based energy economy should also be considered. The large-scale generation of energy from agriculture must be balanced with other critical ecological functions that agricultural lands provide, including nutrient and water cycling, C sequestration, and the maintenance of soil quality. Harvesting the majority of crop biomass from current cropping systems may incur substantial environmental costs. Put simply, removal of the soil cover, nutrients, and C contributed by or contained within crop biomass has the potential to exacerbate the ongoing degradation of soil and water resources by already intensive agricultural production practices (Mann et al., 2002; Lal, 2005; Powers, 2005). To ensure that advanced biofuel technologies are sustainable over the long term, there is an urgent need to develop production systems that are able to generate large quantities of biomass in an economically efficient manner, while also preserving the natural resource base underlying a sustainable bioeconomy.
We propose that the development of new feedstock crops and cropping systems, in conjunction with nutrient recycling between fields and biorefineries, comprise a key strategy for the sustainable production of liquid fuels and other commodity chemicals derived from plant biomass. Whereas a considerable amount of research has been conducted already addressing the potential environmental impacts and benefits resulting from biofuel production (Sheehan et al., 2003; Kim and Dale, 2005; Farrell et al., 2006), little attention has been paid to the issues of the efficiency and sustainability of nutrient management associated with feedstock production for lignocellulosic systems, or for biofuel systems in general where nutrients are recycled from the coproduct streams. Nonetheless, while the technological innovations necessary for nutrient recycling in the production and conversion of lignocellulosic feedstocks are not yet viable on an industrial scale, we believe they are feasible and should serve as primary objectives for both agricultural scientists and engineers during the design and implementation of a new bioeconomy. Our main purpose here is to examine the potential for nutrient recycling in integrated crop biomass and biofuel production, with special attention to N, a critical nutrient for plant growth whose manufacture as fertilizer constitutes about one-third of on-farm fossil fuel inputs (Smil, 2001).
We proceed with an examination of integrated crop–livestock production as an historical example of a very successful coupled feedstock-conversion system and analog for sustainable biofuel production. An overview of lignocellulosic conversion is followed by a general overview of methods of nutrient recovery during conversion. An illustrative example is presented that characterizes N cycling in a representative switchgrass-based ethanol production system that incorporates NH3 recovery and provides a means of evaluating nutrient cycling potential. We conclude with a discussion of alternative crops and cropping systems that exhibit efficient nutrient utilization and are thus complementary to nutrient recycling from conversion processes and that can improve the overall sustainability and production efficiency of the biorefinery system.
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INTEGRATED CROP–LIVESTOCK PRODUCTION: AN ANALOG FOR SUSTAINABLE BIOFUEL PRODUCTION? |
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TOPNOTESABSTRACTINTRODUCTIONINTEGRATED CROP-LIVESTOCK...LIGNOCELLULOSIC BIOMASS...NUTRIENT RECOVERY IN BIOMASS...NITROGEN RECOVERY POTENTIAL: AN...NEW BIOENERGY CROPS AND...CONCLUSIONSREFERENCES |
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By the end of the nineteenth century, these improvements in soil fertility and nutrient cycling between crops and livestock led to nearly a tripling of English cereal yields (Fig. 1 , Grigg, 1982). Although these yield increases were minor in comparison to the Green Revolution that would follow, it is important to note that English cropping systems received few if any external fertility inputs before the 1950s. Integrated crop–livestock systems remained widespread in northwestern Europe, England, and much of the humid, temperate regions of North America until the 1950s and 1960s, when increased availability of relatively low-cost synthetic fertilizers made mixed farming and nutrient recycling biologically unnecessary, and specialized crop and livestock production more economically attractive (Grigg, 1989). In recent years, however, there has been renewed interest in reintegrating crop and livestock systems as a strategy for reducing reliance on fossil fuel resources, minimizing the use of increasingly expensive fertilizers, and limiting water pollution by nutrients, pathogens, and antibiotics (Schiere and Kater, 2001; Naylor et al., 2005). Although integrated crop–livestock systems are much more efficient at recycling nutrients, the wider adoption of such systems is limited by the advantages gained by specialization, allowing the more efficient management of labor and capital resources.
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LIGNOCELLULOSIC BIOMASS CONVERSION |
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TOPNOTESABSTRACTINTRODUCTIONINTEGRATED CROP-LIVESTOCK...LIGNOCELLULOSIC BIOMASS...NUTRIENT RECOVERY IN BIOMASS...NITROGEN RECOVERY POTENTIAL: AN...NEW BIOENERGY CROPS AND...CONCLUSIONSREFERENCES |
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Lignocellulosic biomass is a complex mixture of cellulose, hemicellulose, lignin, and extractives. Bioconversion of lignocellulosic biomass to ethanol consists of four major steps: pretreatment, hydrolysis, fermentation, and separation–purification (typically via distillation). Pretreatment involves converting the structure and chemical composition of recalcitrant biomass so that hydrolysis of the carbohydrates into simple sugars can be achieved more rapidly and efficiently. During hydrolysis, cellulose is broken down into glucose, and hemicellulose is converted to a mixture of soluble five-C and six-C sugars. Enzymatic hydrolysis has been extensively developed because it is seen as providing significant opportunities for improving process yields and lowering ethanol cost relative to other methods (Wyman, 1999). During fermentation, the simple sugars are biologically converted to ethanol or other products. The economics of biomass conversion are significantly improved by utilization of both the five- and six-C sugars, particularly xylose and arabinose that generally comprise a significant fraction of agricultural residues and grasses (Lynd et al., 1999). Strains of bacteria and yeast capable of cofermenting both pentoses and hexoses have been produced through genetic modification (Ingram et al., 1999; Ho et al., 1999). Processes in which saccharification and fermentation are performed in the same reactor are known as simultaneous saccharification and fermentation (SSF) or simultaneous saccharification and cofermentation (SSCF), and result in lower process costs.
Lignin is a branched heterogeneous aromatic polymer that makes up 5 to 20% (dry wt.) of lignocellulosic biomass. It is resistant to biological degradation but is energy rich, with an energy content of around 25 MJ kg–1, a value similar to that of bituminous coal (the most common fuel for power plants). Thus lignin is an important coproduct that can be used to produce more energy than is required to drive the ethanol conversion processes. For this reason, plans for industrial-scale lignocellulosic biomass-to-ethanol conversion plants (i.e., "biorefineries") usually involve an integrated biological and thermochemical process in which the highly ligneous fermentation byproduct is burned or gasified. This combustion or gasification can produce all of the heat required for ethanol distillation and generate electricity to operate the production process, with excess electricity available to sell to other users (McAloon et al., 2000).
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NUTRIENT RECOVERY IN BIOMASS CONVERSION PROCESSES |
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TOPNOTESABSTRACTINTRODUCTIONINTEGRATED CROP-LIVESTOCK...LIGNOCELLULOSIC BIOMASS...NUTRIENT RECOVERY IN BIOMASS...NITROGEN RECOVERY POTENTIAL: AN...NEW BIOENERGY CROPS AND...CONCLUSIONSREFERENCES |
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Recovery of NH3 gas has not yet been demonstrated in thermochemical conversion systems using biomass. However, Sasol Technology of South Africa has an operating commercial process for NH3 recovery from gasified coal (van Nierop et al., 2000). The recovery of gaseous NH3 from a gasifier product stream is technically feasible. Cycling captured NH3 back to production fields has the potential to close an important nutrient cycle; by reducing the need for synthetic N fertilizer inputs, NH3 recyling could also significantly increase the energetic efficiency of biofuel feedstock production.
Ammonia Recovery Technology
Ammonia in the gasifier product gas could be recovered in several ways. The most commonly used industrial methods for gaseous NH3 capture are absorption and mechanical refrigeration. In practice, the choice of recovery technology will be made by examining the fixed and operating costs. Typically, refrigeration is more economically attractive at pressures of 10 MPa (
100 atmospheres) or greater. At lower pressures, absorption–distillation is usually favored (Stocchi, 1990).
Absorption involves a packed-bed column in which the gas stream is put in direct contact with water, and the column is commonly designed to operate countercurrent (liquid and gas moving in opposite directions). Since NH3 is highly soluble in water, recovery efficiencies can be quite high, with recovery up to 99% demonstrated in practical applications (Buonicore and Davis, 1992). Up to recovery efficiencies of 90%, a "wet scrubber" absorption system design is straightforward and does not require a water pH control system; such systems are common in industrial and wastewater treatment applications.
Absorption in a packed-bed column (i.e., a "wet scrubber") will have energy and fixed costs that are nearly independent of operating pressure. Other advantages of a wet scrubber include relatively small space requirements, lower capital cost, and the ability to handle high-humidity and high-temperature gas streams. Ammonia recovery efficiency is improved, however, by reducing the temperature of the synthesis gas leaving the biomass gasifier. Cooling of the gas stream consumes energy and thus reduces overall system power production capacity (e.g., reducing electricity produced from the synthesis gas). Thus there is a trade-off between NH3 recovery efficiency and overall plant energy efficiency. Our preliminary analysis using a process simulation model indicates that the lost electrical generation capacity associated with NH3 recovery should be <2% of the energy content of the feedstock on a lower heat value basis.
To produce highly concentrated aqueous NH3, absorption must be coupled with energy-intensive distillation that will increase fixed costs as well as process complexity. A significant amount of NH3 concentration may be possible with little energy cost, however, by using low-quality waste heat available in the biorefinery process (i.e., through "heat integration"). In practice, the degree of NH3 concentration required will be determined by the requirements of the fertilizer application equipment and transportation and recovery costs.
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NITROGEN RECOVERY POTENTIAL: AN ILLUSTRATIVE EXAMPLE |
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TOPNOTESABSTRACTINTRODUCTIONINTEGRATED CROP-LIVESTOCK...LIGNOCELLULOSIC BIOMASS...NUTRIENT RECOVERY IN BIOMASS...NITROGEN RECOVERY POTENTIAL: AN...NEW BIOENERGY CROPS AND...CONCLUSIONSREFERENCES |
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In this example, we make a preliminary estimate of the N recovery that is possible in converting switchgrass to ethanol and place it in the context of crop N fertilizer requirements. Presented in Table 1 are simple N balances for three possible ethanol-producing systems: corn for grain, corn for grain with stover collection, and switchgrass. The three N balances represent biomass grown on a representative farm in central Iowa (Story County). Corn grain and stover are produced in rotation with soybean [Glycine max (L.) Merr.]. Corn grain and stover yields are based on the Story County, IA, 2002 crop year average of 10.796 Mg ha–1 yr–1 (National Agricultural Statistics Service, 2002) and a harvest index of 0.50 (Linden et al., 2000).
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Maximum sustainable stover harvest is uncertain and will be strongly influenced by soil type and slope as well as soil organic C dynamics and storage (Mann et al., 2002). On relatively flat fields (i.e., 0–2% slope), stover collection beyond 50% may be acceptable from an erosion standpoint (Hasche et al., 2003); however, stover removal may reduce soil organic matter levels unless the cropping system can maintain a net positive or neutral C balance after stover removal. Collection efficiency of corn stover using a multiple-pass system of shredding, windrowing, and baling has been estimated at between 30 and 42% (Richey et al., 1982; Sokhansanj and Turhollow, 2002). Based on more recent work with single-pass stover harvest systems, stover harvest efficiency of 50% is possible (Shinners et al., 2005). We assume a stover harvest efficiency of 50%.
Switchgrass yield is assumed to be 13.5 Mg ha–1 yr–1 based on recent field experiments in Story Counta, IA (Heggenstaller and Moore, unpublished data, 2005). Based on earlier experiments run in Mead, NE, and Ames, IA, Vogel et al. (2002) found that high yields of switchgrass (10.5–11.2 Mg ha–1 yr–1 in Nebraska, 11.1–12.6 Mg ha–1 yr–1 in Iowa) required at least 120 kg N ha–1 yr–1 of fertilizer and recommended application of 10 to 12 kg N ha–1 Mg–1 biomass ha–1. We assume a similar application rate of 142 kg N ha–1 yr–1 as NH4NO3 (i.e., 10.5 kg N ha–1 Mg–1 switchgrass removed). Note that none of the example scenarios evaluated include the additional N application that would be required to compensate for leaching or percolation losses (which are not included in this balance). Sanderson et al. (1997) found switchgrass baling and handling losses of <10%. We assume 10% losses, resulting in switchgrass harvest yield of 12.15 Mg ha–1 yr–1.
Each balance in Table 1 has been developed so that N import and export from the system are equal and long-term soil N can be assumed to be constant (neglecting N losses due to leaching, percolation, and runoff). The balances thus represent the sustainable management of soil N. As shown in Table 1, when harvesting corn stover in addition to grain, additional N must be added to the system to compensate for that removed with the stover. However, N volatilization increases with N fertilizer application rate (Meisinger and Randall, 1991; Schjùrring et al., 1998), so the increment of N addition required to balance the system is greater than the N removed with the corn stover. With the harvest of 50% of the corn stover, N fertilizer application thus increases from 149 to 210 kg ha–1 yr–1, or 41%.
With stover harvest, however, comes the opportunity to recover N during stover biomass conversion. Using an overall recovery efficiency of 82% of N that enters the biomass conversion process (as predicted by process simulation), N recovery is 49 kg N ha–1 yr–1 or 23% of the N applied as fertilizer. There are, of course, significant potential problems associated with removing large fractions of annual crop residues, although corn silage cropping systems have been successfully sustained for many years at similar or greater biomass harvest fractions (Reicosky et al., 2002). More efficient N use by perennials can potentially reduce the environmental impacts associated with nutrient losses and nonrenewable energy input as fertilizer.
Although losses of N through leaching, percolation, and runoff were intentionally not included in these analyses, there are likely to be significant differences between the corn and switchgrass systems that should be noted. Leaching losses are likely to be much higher for corn grain and corn biomass systems than for the switchgrass system (see Randall et al., 1997). From a water quality standpoint, this difference among the systems is not trivial. Randall et al. (1997) found that greater drain flows and NO3–N concentrations in corn–corn and corn–soybean rotations produced annual losses of NO3–N that were 35 times (average loss of 53 kg N ha–1) greater than NO3–N losses in perennial systems (average loss of 1.5 kg N ha–1). Note in Table 1 that switchgrass has lower N loss as NH3 volatilization during senescence resulting from retranslocation of N to belowground rhizomes and roots during shoot senescence (Heckathorn and DeLucia, 1995, 1996).
In the switchgrass scenario in Table 1, the biomass harvest yield of 12 Mg ha–1 yr–1 is 25% less than the total biomass harvest of the corn grain plus stover case (i.e., 16 Mg ha–1 yr–1), but N fertilizer input is 33% less at 142 kg ha–1 yr–1. In addition, because all of the switchgrass biomass is directed to the biorefinery, 111 kg ha–1 yr–1 of N can be recovered. If this N were not recovered, it would be vented to the atmosphere in the form of NH3 gas and oxides of N, contributing to local air and water pollution. The recovered N is equivalent to 78% of the N fertilizer input required. This level of N recovery and cycling could significantly improve the sustainability of biomass production relative to a similar system that did not recover nutrients, and also improve the overall energy balance of ethanol production from lignocellulosic biomass.
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NEW BIOENERGY CROPS AND CROPPING SYSTEMS |
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TOPNOTESABSTRACTINTRODUCTIONINTEGRATED CROP-LIVESTOCK...LIGNOCELLULOSIC BIOMASS...NUTRIENT RECOVERY IN BIOMASS...NITROGEN RECOVERY POTENTIAL: AN...NEW BIOENERGY CROPS AND...CONCLUSIONSREFERENCES |
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In another parallel to the English agricultural revolution of the seventeenth to nineteenth centuries, a new agricultural revolution driven by biorefining may bring with it the introduction of new crops and cropping systems that enhance resource use efficiency as well as the environmental profile of agriculture. A cropping system designed to maximize total biomass production while reducing environmental impacts would probably look very much different than current annual grain-based agriculture.
In the illustrative example developed above, switchgrass was included as a dedicated feedstock crop because it is widely suggested as being well suited to such a role. Although switchgrass is widely adapted geographically, it may not represent the ideal biomass crop for any given location or management scenario (Madakadze et al., 1998; Heaton et al., 2004). Little research has focused on side-by-side comparisons of productivity, C storage, or resource use efficiency by mature stands of different high-yielding perennial grasses (Lemus and Lal, 2005). Furthermore, there is a general lack of information regarding the responses of perennial grass production systems to management practices, including more intensive fertilization practices (Sanderson et al., 1996) and alternate harvest regimes (Mulkey et al., 2006), that are likely to accompany the production of biomass feedstocks for new energy markets (Vogel et al., 2002).
Although perennial crops use nutrients efficiently and provide year-round groundcover, current management technologies render them largely incompatible with grain-based annual cropping systems. With the exception of cool-season hay crops, little research to date has focused on developing management systems that would allow the rotation of annual grain crops with perennial feedstock crops. Switchgrass and other perennial grasses can be established within a corn crop (Hintz et al., 1998), not only allowing the potential to rotate the same land between grain and feedstock production, but also eliminating the traditional loss of production during the year of perennial crop establishment. Given an appropriate level of management, rotations including both perennial and annual crops could support the production of biomass and grain while maintaining soil quality over the long term on high-quality agricultural lands.
Existing summer annuals, including corn, grow for only several months per year and are unable to take full advantage of available sunlight, nutrients, and other resources required for biomass production. Moreover, because summer annual crops provide full canopy cover for only a portion of the season, fields are more susceptible to soil erosion. If residues from annual grain crops are to be sustainably incorporated into a biomass feedstock supply system, then strategies should be developed to ensure that an adequate amount of soil cover is present on crop fields before and after residue removal. The use of a living mulch system could help to protect soil in the context of corn production with stover removal, with the mulch itself providing an early-season biomass harvest. Recent successes with legume-based mulch systems for herbicide-resistant corn production suggest that such an approach may be feasible under a range of conditions (Zemenchik et al., 2000; Affeldt et al., 2004; Duiker and Hartwig, 2004).
Double-crop sequences of annual biomass crops could also be used to improve feedstock production while overcoming many of the liabilities associated with the collection of grain crop residues (Buxton et al., 1999; Karpenstein-Machan, 2001). By pairing a cool-season cover crop with warm-season crops, dedicated biomass double-crop sequences could provide soil cover for most of the year, offering the potential for increased biomass yields and reduced soil and nutrient losses compared with annual-crop monocultures. Biomass double crops including legumes could also contribute to reduced fertilizer N requirements relative to grain crops, thus resulting in the production of biomass with less energy. Extending the current two-crop summer annual system (e.g., corn–soybean) to include double crops could offer a near-term approach for linking the emergence of new markets for biomass with the sustainable diversification of grain-based farming systems.
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CONCLUSIONS |
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TOPNOTESABSTRACTINTRODUCTIONINTEGRATED CROP-LIVESTOCK...LIGNOCELLULOSIC BIOMASS...NUTRIENT RECOVERY IN BIOMASS...NITROGEN RECOVERY POTENTIAL: AN...NEW BIOENERGY CROPS AND...CONCLUSIONSREFERENCES |
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ACKNOWLEDGMENTS |
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NOTES |
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TOPNOTESABSTRACTINTRODUCTIONINTEGRATED CROP-LIVESTOCK...LIGNOCELLULOSIC BIOMASS...NUTRIENT RECOVERY IN BIOMASS...NITROGEN RECOVERY POTENTIAL: AN...NEW BIOENERGY CROPS AND...CONCLUSIONSREFERENCES |
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Received for publication June 22, 2006.
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REFERENCES |
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TOPNOTESABSTRACTINTRODUCTIONINTEGRATED CROP-LIVESTOCK...LIGNOCELLULOSIC BIOMASS...NUTRIENT RECOVERY IN BIOMASS...NITROGEN RECOVERY POTENTIAL: AN...NEW BIOENERGY CROPS AND...CONCLUSIONSREFERENCES |
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