Harvest and Conversion Systems for Producing Energy from Switchgrass: Logistic and Economic Considerations

David I. Bransby, Professor, Department of Agronomy and Soils, Auburn University, Auburn, AL 36849; Roger Samson, Executive Director, REAP-Canada, Box 125 Centennial Centre CCB13, Ste. Anne de Bellevue, QC H9X 3V9; and David J. Parrish, Professor, and John H. Fike, Associate Professor, Department of Crop and Soil Environmental Sciences, Virginia Tech, Blacksburg, VA 24061

Abstract

Switchgrass (Panicum virgatum L.) offers considerable potential as a feedstock for production of bioenergy and bioproducts. Consequently, a substantial amount of research has been conducted on genetic and agronomic aspects of switchgrass as an energy feedstock. However, only a limited amount of work has been conducted on developing harvest and conversion systems for producing energy from switchgrass, including logistical and economic considerations. Our objective was to review this work, with an emphasis on switchgrass composition, conversion technologies, methods for harvesting, storing, compacting and transporting switchgrass, government incentives, commercial systems, and economics. Switchgrass can be converted into heat, electricity, gas, and liquid fuels. Heat, electricity, and gas production at both small and large scales are generally achieved by combustion or gasification. A wide range of liquid fuels (including ethanol, butanol, diesel, and gasoline) can be produced from switchgrass by means of various technologies. These can be categorized mainly into two groups: biochemical and thermochemical. Switchgrass can be baled, field chopped, or pelletized prior to storage and delivery to processing plants. Government incentives and carbon markets should facilitate initial commercialization of switchgrass to produce energy in the next 3 to 5 years, with large scale expansion starting in 5 to 10 years.

Introduction

Coordinated research on herbaceous energy crops was initiated by the United States Department of Energy (DOE) in 1985. Early work involved screening a number of herbaceous species across a wide range of soil and climatic conditions in the eastern USA. Following these initial studies, more concentrated efforts began in 1992 in North America to focus on the physiology, genetics, and agronomy of switchgrass (7). Results of this work have been widely reported (8) and include a recent review in Forages and Grazinglands (9). During the 1990s, the relatively low price of fossil fuels was a major barrier to commercialization of switchgrass for energy. However, in recent years a sharp increase in the price of crude oil and natural gas, instability in several major oil producing countries, and greater global concern about the impact of fossil fuels on increased greenhouse gases and the associated risk of global climate change have substantially raised interest in all forms of renewable energy.

Switchgrass, along with many other forms of plant biomass, can be converted into heat, electricity, gas, and liquid fuels. To develop systems for production of these forms of energy on a commercial scale, logistics for harvesting, storage, pre-processing, and transport need to be developed and optimized, but relatively little work has been focused on these issues. While the handling of biomass to produce energy has some similarities to forage systems, some distinct differences are also involved. Therefore, our objectives are to review: (i) the composition of switchgrass as it relates to energy production; (ii) technologies for conversion of switchgrass to different forms of energy; (iii) methods for harvesting, storing, compacting, and transporting switchgrass; and (iv) government incentives, systems, and economics for supplying switchgrass to bio-processing facilities.

Switchgrass Composition

As is the case for forage that will be fed to livestock, the composition of biomass feedstocks for production of energy is important, and needs to be optimized for the conversion or processing technology. Major constituents in switchgrass include ash, cellulose, hemicellulose, lignin, and moisture. Switchgrass typically contains 3 to 6% ash, 30 to 34% cellulose, 24 to 27% hemicellulose, 16 to 19% Klasson lignin, and about 8,000 Btu of energy per pound on a dry basis. It can usually be dried to 10 to 15% moisture in the field within a few days of mowing. Variation in these major components is relatively low among cultivars. In contrast, woody biomass such as that from hybrid poplar, is generally lower in ash (1 to 2%) and hemicellulose (16 to 19%) but higher in cellulose (40 to 43%), lignin (22 to 25%), energy (~8,400 Btu/lb), and moisture (40 to 45%) than switchgrass. Both switchgrass and wood generally contain low levels of N and S, resulting in low emission of N and S oxides, which can have negative impacts on the environment and/or human health.

Conversion Technologies

Production of heat is one of the most efficient processes for generating useful energy from biomass: typically it is possible to recover 80% or more of the energy contained in biomass as heat. Both small and large systems are available for producing heat with switchgrass. Pellet stoves are used for home heating systems, and small furnaces or gasifiers are used for applications such as heating industrial buildings or broiler houses. Because combustion occurs with limited or no restriction of air (and therefore, oxygen) to the furnace, relatively high temperatures (> 2,500°F) are involved. If combustion is complete, the products are mainly water vapor, CO₂ and ash. However, depending on feedstock composition, combustion may also result in release of gases such as N and S oxides which are harmful to the environment.

Electrical power can also be generated from biomass in both small and large systems. An example of a small biomass power system would be use of a gasifier to produce synthesis gas, or syngas (mainly CO and H, but also including small amounts of CH₄ and CO₂), which then powers an internal combustion engine to drive an electrical generator. However, syngas typically only contains about 400 Btu/ftł compared to about 1,000 Btu/ftł for natural gas. On a larger scale, biomass can be used in a furnace or gasifier to generate steam, which then drives a turbine. This fundamental process is used in most pulp mills to generate power from mill residues and is also the basic method used to generate about half the electrical power in the USA from burning coal.

In typical coal-fired power plants, co-injecting the coal and biomass (a process known as co-firing) offers immediate opportunities to use biomass for electricity production. Trials for co-firing up to 10% switchgrass with coal in existing power plants have been conducted in Alabama (1) and Iowa. Co-firing higher proportions of switchgrass with coal is difficult because of its relatively low bulk density: 10% switchgrass by weight amounts to about 50% by volume because the bulk density of switchgrass is so much lower than that of coal. Low bulk density also necessitates separate handling of the two feedstocks. Coal is fed by gravity feeding systems to a pulverizer, and the resultant powder is delivered pneumatically to boilers. However, chopped switchgrass tends to bridge in gravity-feeding systems, causing blockages, and thus needs to be injected into the boiler separately from the coal. Co-firing switchgrass results in a reduction in undesirable emissions but is more expensive than burning only coal on an energy equivalent basis.

A number of liquid fuels can be produced from switchgrass. Cellulosic ethanol is perhaps the most widely promoted liquid fuel. It can be produced by either of two primary pathways: biochemical or thermochemical (4,5). The biochemical pathway has received more research attention than the thermochemical pathway. It involves enzymatic or acid hydrolysis of the cellulose and hemicellulose into component sugars, fermentation of the sugars into ethanol, and separation of the ethanol from the resulting beer by distillation. Typically, a pretreatment step is required to reduce feedstock recalcitrance (masking or binding of the cellulose and hemicellulose by lignin) to conversion. Most current pretreatment processes involve steam explosion or application of ammonia to break apart the feedstock fibers. Cellulose is more difficult to hydrolyze than hemicellulose, but the five-C sugars derived from hemicellulose are more difficult to ferment. Expected yields from biochemical processes are 60 to 80 gal ethanol per dry ton of biomass. Due to lower lignin and higher cellulose and hemicellulose contents in grasses, ethanol yields are expected to be higher than for wood.

Thermochemical conversion involves gasification of the biomass into syngas and conversion of the syngas to liquid fuel by catalysts under high pressure and high temperature in catalytic reactors (4,10). These fuels can be ethanol, methanol, and/or diesel, depending on the catalyst used. Thermochemical technology is more flexible with respect to feedstock, and expected yields are above 100 gal/ton of biomass. However, a recent analysis found no economic advantage over the biochemical option (14).

A hybrid thermochemical-biochemical system can also be used in which the syngas from the gasifier is converted to ethanol by microorganisms or biocatalysts instead of chemical catalysts. Yet another version of the thermochemical process involves catalytic degradation or depolymerization of polymers, followed by catalytic synthesis into renewable alkanes that resemble the diesel, gasoline, and aviation fuel that are currently generated from oil (10).

While no economically successful commercial plants that produce transportation fuels from cellulosic feedstocks are in production yet, the US DOE is in the process of assisting with funding for six projects which involve building commercial scale cellulosic ethanol plants. In addition, private companies are proceeding with similar efforts without government assistance. Therefore, it is likely that the first plants of this kind will be in operation within the next 3 to 5 years, and substantial expansion of the industry will occur in 5 to 10 years.

Harvest and Post-Harvest Logistics

Green switchgrass is best cut with a mower-conditioner instead of a regular mower to ensure rapid drying. Once mown, raking switchgrass into windrows can accelerate drying, but in fields with high yields of tall grass this can create difficulty for subsequent chopping or baling operations: windrows may be too large for proper handling with existing hay making equipment that is designed for lower yields, and in such cases it might be best to simply let the material dry in the mown swath instead of raking, even if drying takes longer. Once dry, the biomass may be baled with a big round or big square baler. However, on sites with steep topography, big square balers may not be feasible. Round bales may also present certain advantages in humid sites because they can shed water, reducing the need for covered storage. As an alternative to baling, mown switchgrass can be directly chopped in the field with either pull-behind or self-propelled silage choppers with pick-up heads attached. Because the material is dry, this usually requires sharpening blades more often than when wet material like silage is chopped. Ideally, the objective is to achieve a particle size of about half an inch. While chopping might be slower than round baling in the field, chopped material can be loaded and unloaded in less time than it takes to load round bales, and in-field chopping eliminates the need for tub grinding prior to feeding the material into a processing plant.

Principles related to storing switchgrass bales for production of energy are the same as for storing bales of hay: moisture causes damage and loss of dry matter. Uniformly dry material needs to be put into storage to reduce these risks and the risk of fire. It is best to store material under a roof or with a perforated tarp that limits condensation of moisture on the under side of the cover, particularly for large square bales. If this is not possible, bales can be stored outside, preferably on well-drained gravel to prevent contact with soil, and well spaced to allow adequate air movement among bales for drying following rain. Chopped switchgrass is also best preserved if under cover, but the material will stay remarkably well preserved if stored in a pile that is exposed to the weather. If such piles are compressed (by riding on them with a tractor) and care is taken to ensure the sides of the pile are smooth and relatively steep, the surface particles can form a thatch that sheds moisture. Cotton module builders represent another handling option currently being considered for chopped feedstock. Switchgrass can be difficult to pelletize effectively if dies and equipment are not optimized for grass pelleting. In such cases, adding a "binder" might be effective.

The efficient transportation of the biomass will depend on the hauling distance and the local road regulations. For longer distance hauling, high-density 3 × 4 × 8 bales will provide the greatest load. For example using a 53-inch single-drop deck trailer, 50 bales can be transported. Using normal density switchgrass bales, this results in a 22 to 24-ton load. However with high density bales 26 to 28 tons can be put on a load. The bulk density of switchgrass chopped to a particle size of half an inch is 8 to 9 lb/ftł. This results in a load of 12.0 to 13.5 tons on a 42-ft walking floor trailer. Some newer forage harvesters are successfully achieving finer chops from the field, which can result in load densities of up to 20 ton/load. Thus road transport of fine chopped switchgrass can approach low density large square bales.

Government Incentives, Commercial Systems, and Economics

Wide-scale adoption of switchgrass to produce energy will depend largely on developing economically competitive supply and conversion systems (11). As a crop, it will need to compete with traditional crops for land on farms and with other forms of biomass that can be used for the desired conversion process, such as woody biomass and crop residues like corn stover and wheat straw. Another economic hurdle for switchgrass can be relatively low biomass yields in the seeding and second years. On the product end, switchgrass biofuel systems need to be competitive with other biofuels, such as ethanol-from-corn, and with competing fossil fuel products including gasoline, diesel, heating oil, propane, natural gas and coal.

Because many traditional crops enjoy government price-support programs, it may be difficult for energy crops like switchgrass to compete for farmland without such incentives. One way to mitigate this disadvantage would be to provide bridging payments in the first two years following seeding. Alternatively, Conservation Reserve Program (CRP) land could be used to grow and harvest switchgrass for energy without growers forfeiting CRP payments, but this latter option will require cultural management strategies that are compatible with concerns from the environmental and wildlife advocacy communities. Recently approved legislation that should facilitate use of switchgrass as an energy crop in the US is the Energy Independence and Security Act of 2007 which mandates at least 44% of alternative fuels be produced from cellulosic feedstocks. In addition, carbon credit markets are starting to develop, and because switchgrass is effective in sequestering carbon (6), this development should also facilitate commercial production of the crop.

At current input costs, the delivered cost of switchgrass is between $50 and $60 per dry ton over a fairly wide range of conditions. If a $10/ton profit for the grower is added, this would amount to an average delivered price to the processing plant of about $60 to 70/ton. Areas with low land rents will likely be in the lower end of this range and areas with higher land rents could be well above this cost. Successful projects will need to evolve in areas where switchgrass can compete with alternative feedstocks that can be used in the application. In some areas woody biomass and crop residues are currently available at considerably lower prices than this.

Assuming an energy content of 16 million Btu/dry ton of switchgrass, a delivered price of $65/ton amounts to a cost of $4.06/million Btu. In comparison, oil (which contains 5.8 million Btu/barrel) at a price of $140/barrel amounts to an energy cost of $24.14/million Btu. Therefore, on a cost/million Btu basis, switchgrass is extremely competitive with oil as a raw material, and this could partially explain the rapidly increasing interest in cellulosic biofuels among oil companies. However, technology that can convert biomass to liquid fuel as efficiently as oil is converted has still not been developed. The current cost of natural gas is between $11 and $13/million Btu with higher prices in the peak winter heating period of January through March, and that of coal is mostly between $2 and $3/million Btu. The low price of coal explains why utilities are reluctant to co-fire switchgrass with coal without significant government incentives or premium prices for the "green power" produced.

Development of switchgrass for energy production will be dependent upon the value of the feedstock relative to traditional food, feed, and fiber crops. However, biomass cropping system economics may to some degree be buffered from price fluctuations common to other commodities, because processing facilities will likely seek multi-year production contracts to guarantee supply. As energy cropping matures as an agricultural enterprise, new machinery for harvesting and processing biomass will likely be developed; but, in the near term, it is likely that there would be an advantage to using existing forage handling equipment. Bransby et al. (2) developed an interactive budget model to evaluate four systems that could be used immediately in the southeastern USA: (i) traditional mowing and round baling, then hauling bales to the plant where they are pulverized or ground prior to processing; (ii) field chopping following mowing, and transporting chopped material to the processing plant in a walking floor trailer; (iii) field chopping and creating a compacted module with a cotton module builder, thereafter transporting the modules to the processing plant in a cotton module truck; and (iv) field chopping followed by pelletizing and transporting pellets to the plant. The model was then used to examine the effect of switchgrass yield, transport distance, truck capacity, and stand life on delivered cost of biomass to the plant using each of the four systems.

Results of this study indicated that field chopping and hauling chopped material in a walking floor trailer or after it had been compacted with a cotton module builder resulted in lower delivered cost to a large bioenergy conversion facility than round baling or pelletizing. The high cost of baling was related to more individual operations needed in the baling option, while the high cost of the pelletizing system was related to the cost of producing the pellets. As suggested above, chopped switchgrass can be stored in piles that tend to thatch and shed water, resulting in relatively low losses. Switchgrass yield and hauling distance to the plant had greater impacts on delivered cost than stand life and truck capacity. Delivered cost decreased as yield increased, but this effect was not linear, and the response was relatively small above 8 tons/acre. In contrast, delivered cost increased linearly with distance from the processing plant. Delivered cost decreased as truck capacity and stand life increased, but effects were relatively small above a truck capacity of 20 tons and a stand life of 10 years. Breakeven yield was about 4.5 tons/acre, suggesting that the yield of only 1.7 tons/acre measured by Tilman et. al. (13), for polycultures of native grasses in Minnesota is well below the yield range that is of economic relevance.

Development of a viable commercial enterprise that uses switchgrass as a feedstock to produce energy requires a wide range of issues to be considered. These include the amount of switchgrass and land area needed, average hauling distance, and business structure. The challenges related to developing a feedstock supply system are often underestimated, so a specific example of how these issues might be addressed should be useful. The capacity of most corn-ethanol plants ranges from 50 to 100 million gal/year. If a 50-million-gal/year capacity and an efficiency of 80 gal/ton are assumed for a cellulosic processing facility, the plant would need 625,000 tons of switchgrass per year. For 350 days of operation each year, this would amount to 1,786 tons (or 89 truck loads of 20 tons each) per day. At a yield of 5 tons/acre, 125,000 acres of switchgrass would be needed, and if a 10-day supply is needed on site at the conversion plant, storage space is required for 17,860 tons of switchgrass. Assuming the plant was in the center of a circular production area, and assuming the average hauling distance is 20% farther than a direct line from the grower to the plant (to account for curvature of roads) average hauling distance would be 29.9, 21.2, and 15.0 miles if 5, 10, or 20% of the land surrounding the plant was established to switchgrass.

In contrast, bionergy converison technologies such as pellet and biogas plants have a much smaller land footprint requirement. A 150-ton/day pellet plant would require 10,500 acres. A biogas facility which was fed with a diversity of energy resources such as manure and corn silage might utilize only 500 acres of switchgrass. Lower feedstock delivery costs will be realized if successful small to medium scale bioenergy technologies can be commercialized using local bulk handling technologies.

Finally, the issue of energy balance (energy output as a proportion of energy inputs) is often raised in relation to production of biofuels. For corn-to-ethanol this statistic is about 1.3, and surprisingly, for gasoline produced from oil it is negative: 0.81 (3). In contrast, a recent study from the Great Plains (12) indicated that for ethanol produced from switchgrass this figure is 5.4, or alternatively, that 540% more energy was contained in the ethanol produced, than was used in growing the switchgrass and converting it to liquid fuel. Because the emerging bioenergy industry is at a very early stage, it is likely that this figure can be improved substantially by improving crop yield and conversion technology.

Conclusion

As an energy feedstock, compared to wood, switchgrass has the advantage of being easier to dry, but has the disadvantage of being more prone to cause slagging in furnaces and having a higher ash content. On a cost per unit energy basis, the cost of switchgrass is higher than for wood and coal, but only 26% of that for crude oil at $90/barrel. Commercialization of switchgrass for production of energy will require development and optimization of not only agronomic practices, but also harvesting, storage, transport and conversion systems. Procedures and equipment used for these purposes in commercial forage production can be adapted for this purpose, including baling, field chopping and pelletizing.

Technologies are available or are being developed to produce heat, electricity, gas and a range of liquid fuels (including ethanol, butanol, diesel, and gasoline) from cellulosic feedstocks such as switchgrass. These technologies vary somewhat with respect to desirable feedstock characteristics, thus demanding adaptation of agronomic, harvesting, storage and pre-processing procedures to suit their different needs. Opportunities to grow and supply switchgrass for commercial energy production can be expected to develop in the next 5 to 10 years.

Literature Cited

1. Boylan, D., Bush, V., and Bransby, D. I. 1998. Switchgrass cofiring: pilot scale and field evaluation. Biomass Bioenergy 19:411-417.

2. Bransby, D. I., Smith, H. A., Taylor, C. R., and Duffy, P. A. 2005. An interactive budget model for producing and delivering switchgrass to a bioprocessing plant. Indust. Biotech. 1:122-125.

3. Dale, B. E. 2007. Thinking clearly about biofuels: ending the irrelevant net energy debate and developing better performance metrics for alternative fuels. Biofuels Bioprod. Bioref. 1:14-17.

4. Huber, G. W., Iborra, S., and Corma, A. 2006. Synthesis of transportation fuels from biomass: chemistry, catalysts and engineering. Chem. Rev. 106:4044-4098.

5. Lange, J. 2007. Lignocellulose conversion: an introduction to chemistry, process and economics. Biofuels Bioprod. Bioref. 1:39-48.

6. Ma, Z., Wood, C. W., and Bransby, D. I. 2000. Soil management impacts on soil carbon sequestration by switchgrass. Biomass Bioenergy 18:469-477.

7. McLaughlin, S. B., and Kszos, L. A. 2005. Development of switchgrass (Panicum virgatum) as a bioenergy feedstock in the United States. Biomass Bioenergy 28:515-535.

8. Parrish, D. J., and Fike, J. H. 2005. The biology and agronomy of switchgrass for biofuels. Crit. Rev. Plant Sci. 24:423-459.

9. Parrish, D. J., Fike, J. H., Bransby, D. I., and Samson, R. 2008. Establishing and managing switchgrass as an energy crop. Online. Forage and Grazinglands doi:10.1094/FG-2008-0220-01-RV.

10. Pu, Y., Zhang, D., Singh, P., and Ragauskas, A. J. 2008. The new forestry biofuels sector. Biofuels Bioprod. Bioref. 2:58-73.

11. Samson, R., Stamler, S. B., Dooper, J., Mulder, S., Ingram, T., Clark, K., and Lem, C. H. 2008. Analysing Ontario biofuel options: Greenhouse gas mitigation efficiency and costs. REAP-Canada, Centennial Centre CCB13, Sainte Anne de Bellevue, Quebec, H9X3V9, Canada.

12. Schmer, M. R., Vogel, K. P., Mitchell, R. B., and Perrin, R. K. 2008. Net energy of cellulosic ethanol from switchgrass. PNAS 105:464-469.

13. Tilman, T., Hill, J., and Lehman, L. 2006. Carbon-negative biofuels from low-input high-diversity grassland biomass. Science 314:1598-1600.

14. Wright, M. M., and Brown, R. C. 2007. Comparative economics of biorefineries based on the biochemical and thermochemical platforms. Biofuels Bioprod. Bioref. 1:49-56.