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Energy crops such as switchgrass and giant miscanthus can be used to make up the shortfall in biomass feedstock for second generation biofuels

Summary​

Energy crops are plants grown specifically for energy production and the would be required for second generation biofuels, as agricultural and forest residues alone cannot meet the feedstock requirements. These crops are categorized into four types: herbaceous perennial grasses, woody trees/shrubs, annual crops, and non-food oil crops. In the United States, switchgrass and giant miscanthus have been extensively researched for their high biomass yields and environmental benefits. Key environmental considerations for energy crops include potential indirect land use change impacts and ecosystem effects, though perennial crops can provide habitat and erosion control benefits. Collection methods are similar to conventional forage crops but face challenges with bulk volume transport and storage logistics.

Energy crops can be defined as any crop that is grown for the primary purpose of producing energy, along with any co-products that are produced along the way. In the context of second generation biofuels, energy crops are vital as the amount of lignocellulosic biomass available to achieve sustainability goals such as decarbonization of the aviation industry cannot be sustainably supplied from agricultural residues and forest resource residues.

Since the 1970s, energy crops have undergone extensive research in the United States to develop crop management practices and to explore their potential in a number of functions 1. Energy crops can be separated into four categories: herbaceous, woody, annual, and non-food oil crops.

Herbaceous energy crops​

Herbaceous energy crops, also referred to as perennial grasses, are crops that live for more than two years and generally exhibit characteristics such as large root systems, high biomass yields, and reduced fertilizer and herbicide usage 1. Herbaceous energy crops have traditionally been grown since the 1940s as a form of regenerating land that was damaged by drought, or as a form of forage animal feed 1. Examples of herbaceous energy crops include:

  • Switchgrass
  • Giant miscanthus
  • Elephant grass
  • Giant reed
  • Big bluestem

Woody energy crops​

Woody energy crops are essentially trees or shrubs that are grown for the primary purpose of producing energy. Woody energy crops are generally characterized by their ability to rapidly produce high biomass yields. Desirable characteristics of woody energy crops are a short rotation time and low water usage, but the definition of woody energy crops is flexible and can include conventional tree crops such as eucalyptus and pine 2. Examples of woody energy crops include:

  • Willow
  • Poplar
  • Pine
  • Eucalyptus

Annual energy crops​

Annual energy crops are simply annual crops that are grown for the purpose of producing energy. Under this definition, crops such as corn and soybean could be considered annual energy crops; however, in the context of second generation biofuels, annual energy crops tend to be bred in a way that reduces their competition with food crops. One example of an annual energy crop that has been studied extensively is energy sorghum, grown for its high lignocellulosic biomass yields. Sorghum can be grown in a grain variety for its cereal, forage variety for animal feed palatability, and a sweet sorghum variety for its sugar content 1. Interestingly, energy sorghum is technically perennial, but it is generally grown from seed in an annual crop cycle 1. Another example of an annual energy crop is energy cane, which is a high fiber variety of sugarcane that is grown for its high biomass yields instead of its sugar content that can be used as lignocellulosic feedstock 1.

Non-food oil energy crops​

Non-food oil energy crops are crops that can be grown for their high oil content while not directly competing with food crops. The most studied non-food oil is the jatropha, a drought-resistant plant that is native to Mexico that contains a high oil content in its seed (35%–40%) and its kernel (50%–60%). The seeds of the jatropha plant are considered toxic when consumed, meaning that it meets the classification of a non-food crop. Recently, in the United States, more attention has been focused on oilseeds of the Brassica species, which include pennycress, camelina, and carinata. These plants are considered 'intermediate oilseed' crops as they can be grown in between summer crops, thus not impacting indirect land use change while also providing ecosystem benefits 2.

Energy crops in the United States​

In the United States, the dedicated energy crops that have undergone the most research are the perennial grasses switchgrass and giant miscanthus. Switchgrass is a native grass that is broadly adapted to various growing conditions in the United States with high productivity, low water usage, and it can be grown from seed 1. Giant miscanthus (Miscanthus X giganteus) is a hybrid of two grasses that are native to Asia that produces high biomass yields that has only more recently been studied in the United States for its potential as an energy crop. Giant miscanthus is grown from rhizomes, similar to ginger, which are excellent for combating erosion 1. Giant miscanthus requires more water in order to achieve high yields, but its fertilizer requirements are very low, with some studies showing that fertilizer application resulted in no yield increase 1. The largest benefit of giant miscanthus is its sustainability credentials, with low fertilizer requirements and strong soil organic carbon fixation resulting in liquid fuels with very low greenhouse gas emissions, in some cases being negative, meaning that in the lifecycle production of the fuel, more carbon is removed from the atmosphere than is added 3 4. The major downside of giant miscanthus is that because it is grown from a rhizome, extensive and labor-intensive propagation methods are required and rhizomes require sometimes up to 3 years to mature, making commercial cultivation a risky prospect for farmers 1.

Environmental considerations​

The main environmental concern regarding dedicated energy crops is the degree to which they lead to induced land use change. Energy crops like switchgrass and giant miscanthus are often praised for their ability to be grown on marginal or degraded land, and in some cases rehabilitating the land that they are grown on 5. Although this is true, yields on marginal land are typically lower than on productive land, meaning that if a market was created and sufficient financial incentives existed, the energy crop would likely be grown on productive land, consequently leading to indirect land use change 6. If indirect land use change does occur, the carbon emission benefits of the energy crop can reduce significantly.

In addition to land use change, energy crops can also affect the ecosystem and biodiversity. It is acknowledged that energy crops, specifically perennials, can provide some ecosystem benefits such as beneficial wildlife habitat for bugs and insects, erosion control, landscape diversity, and in some cases improvement of air and water quality 1. However, many benefits are reduced when energy crops are grown as a monocrop (monocultures) as would be necessary to meet the demands of a commercial second generation biofuels program. Monocultures of switchgrass can increase the potential for invasive species, and accelerate pest spread, requiring more pesticides 7.

Collection of energy crops​

Collection of energy crops is not too dissimilar to the collection of other forage crops used in the dairy industry such as alfalfa and hay, with a range of commercially available farm equipment available for harvesting and baling 1. Energy crops still do present unique challenges as a feedstock, such as large bulk volume which can cause logistical challenges in transport, storage, and processing 1 8. Again, the sheer scale of biomass that may be required is a challenge in itself, meaning that other processes that can increase flowability and storage capabilities such as pelletization may be required to reduce costs and maintain profitability 8.

For detailed breakdown of switchgrass, refer to the following sections:

Sources​

Footnotes​

  1. Downing, M., Eaton, L. M., Graham, R. L., Langholtz, M. H., Perlack, R. D., Turhollow, A. F., ... & Brandt, C. C. (2011). U.S. Billion-Ton Update: Biomass supply for a bioenergy and bioproducts industry (No. ORNL/TM-2011/224). Oak Ridge National Laboratory. ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7 ↩8 ↩9 ↩10 ↩11 ↩12 ↩13

  2. Langholtz, M. H. (Lead). (2024). 2023 Billion-Ton Report: An assessment of U.S. renewable carbon resources (ORNL/SPR-2024/3103). Oak Ridge National Laboratory. https://doi.org/10.23720/BT2023/2316165 ↩ ↩2

  3. Kurzawska-Pietrowicz, P. (2023). Life Cycle emission of selected Sustainable Aviation Fuels – A review. Transportation Research Procedia, 75, 77–85. https://doi.org/10.1016/j.trpro.2023.12.010 ↩

  4. Detsios, N., Theodoraki, S., Maragoudaki, L., Atsonios, K., Grammelis, P., & Orfanoudakis, N. G. (2023). Recent Advances on Alternative Aviation Fuels/Pathways: A Critical Review. Energies, 16(4), 1904. https://doi.org/10.3390/en16041904 ↩

  5. Zumkehr, A., & Campbell, J. E. (2013). Historical U.S. cropland areas and the potential for bioenergy production on abandoned croplands. Environmental Science & Technology, 47(8), 3840–3847. https://doi.org/10.1021/es3033132 ↩

  6. Sands, R. D., Malcolm, S. A., Suttles, S. A., & Marshall, E. (2017). Dedicated Energy Crops and Competition for Agricultural Land (ERR-223). U.S. Department of Agriculture, Economic Research Service. ↩

  7. Hartman, J. C., Nippert, J. B., Orozco, R. A., & Springer, C. J. (2011). Potential ecological impacts of switchgrass (Panicum virgatum L.) biofuel cultivation in the Central Great Plains, USA. Biomass and Bioenergy, 35(8), 3415–3421. https://doi.org/10.1016/j.biombioe.2011.04.055 ↩

  8. Hess, J. R., Wright, C. T., & Kenney, K. L. (2007). Cellulosic biomass feedstocks and logistics for ethanol production. Biofuels, Bioproducts and Biorefining, 1(2-3), 181–190. https://doi.org/10.1002/bbb.26 ↩ ↩2