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Switchgrass and other lignocellulosic biomass faces logistical concerns related to low bulk density and poor flowability which could be solved by pelletization

Switchgrass and other lignocellulosic biomass have low bulk density, creating storage and transportation challenges for large-scale biofuel production. Pelletization addresses these issues by compressing biomass into dense, flowable pellets through a two-step process involving size reduction and compression through a die. While pelletization offers significant benefits for handling and transport, it comes with challenges including energy costs. Lignin content in biomass acts as a natural binder during pelletization but can interfere with bioethanol production efficiency. The US wood pellet industry is an example of a large pelletization industry that has grown significantly to meet European demand, reaching nearly 9 million tons by 2022, though this expansion has raised numerous ecological and community health concerns.

Switchgrass, along with other lignocellulosic biomass such as agricultural residues and forest residues, naturally tends to have a low bulk density. Essentially, this means that without some form of processing, bulk volumes of the biomass material tend to contain a lot of empty space. This is a major hurdle for any large-scale second generation biofuels industry as it introduces inefficiencies in storage and transportation. Feedstock with low bulk density such as baled switchgrass also has very poor flowability, meaning that it cannot easily be gravity-fed into processing equipment and it is susceptible to clogging. This differs from the production of corn ethanol, where individual heads of corn are dried in the field, after which the individual kernels are removed off the cob where they are already in a low-volume state with easy flowability.

The primary method proposed to address these logistical issues is pelletization, where biomass is compressed into small, dense, and flowable pellets that can be easily stored and transported.

What does pelletization look like?​

Pelletization, sometimes called mechanical densification, is a two-step process first requiring a rough size reduction step (such as a hammer mill) so that the biomass can be fed into the pellet mill for pelletization. Pellet mills operate by compressing the size-reduced biomass through a die (somewhat like a pasta machine used to make macaroni), after which a blade chops the pellets to the desired size. Sometimes, there is an additional conditioning step (often with steam) within the pellet mill to soften up the biomass so that it can be more readily compressed.

Challenges with pelletization​

Pelletization offers enormous benefits to the prospects of utilizing lignocellulosic biomass; however, there are a number of challenges with pelletization that would need to be overcome to make it viable. The monetary cost of pelletization is estimated to be around $30 per ton, compared to $20 per ton for regular baling 1. But this added cost may be offset by the cost savings in storage, transportation, and handling of the material.

Energy costs​

Pelletization is generally a highly energy-intensive process, as hammer and pellet mills operate via rotating heavy-massed objects at high speeds. Assuming that electric motors are used in the pelletization process, the energy requirements for the densification of one ton of switchgrass are estimated to be around 146 kWh (525.6 MJ) 2. If switchgrass production was scaled up to the level required to achieve the goal of decarbonization of the US aviation industry, this would be a very significant energy cost.

Lignin Issues and pellet durability​

For pelletization to be effective, pellets require a degree of durability so that they do not break down into a fine dust when transported. For the pelletization of lignocellulosic biomass, the lignin content in the biomass normally acts as an effective binder and no extra binder additive is required. Lignin, a naturally occurring complex polymer, is activated in warm environments, such as in the presence of heated steam in the pellet mill conditioning step where it plasticizes and migrates to the surface of the biomass, binding it together 3. However, in bioethanol production, lignin is considered an undesirable product as it inhibits the enzymatic breakdown of cellulosic material and usually needs to be removed through various pretreatment steps 4. Nonetheless, pretreatment is essentially a prerequisite for processing all lignocellulosic biomass, and depending on the pretreatment method, pelletization can have no negative impact on the bioethanol production process 5.

Scaling up pelletization​

The United States already has a robust and expanding market for pelletization, particularly for wood pellets that are sent to the European heating and energy markets. This growth was primarily due to the European Union classifying biomass as a renewable energy source, which skyrocketed US export demand 6. Between 2009 and 2022, the United States wood pellet export capacity grew from 300,000 tons to nearly 9 million tons 6 7. This growth has been accompanied by significant ecological issues and health concerns for the communities located nearby to pelletizing facilities 6. The global wood pellet market is estimated to account for over 60 million tons as of 2025 8.

Sources​

Footnotes​

  1. 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. Crawford, N. C., Ray, A. E., Yancey, N. A., & Nagle, N. (2015). Evaluating the pelletization of "pure" and blended lignocellulosic biomass feedstocks. Fuel Processing Technology, 140, 46-56. https://doi.org/10.1016/j.fuproc.2015.08.023 ↩

  3. Whittaker, C., & Shield, I. (2017). Factors affecting wood, energy grass and straw pellet durability – A review. Renewable and Sustainable Energy Reviews, 71, 1-11. https://doi.org/10.1016/j.rser.2016.12.119 ↩

  4. Naik, S. N., Goud, V. V., Rout, P. K., & Dalai, A. K. (2010). Production of first and second generation biofuels: A comprehensive review. Renewable and Sustainable Energy Reviews, 14(2), 578–597. https://doi.org/10.1016/j.rser.2009.10.003 ↩

  5. Rijal, B., Igathinathane, C., Karki, B., Yu, M., & Pryor, S. W. (2012). Combined effect of pelleting and pretreatment on enzymatic hydrolysis of switchgrass. Bioresource Technology, 116, 36-41. https://doi.org/10.1016/j.biortech.2012.04.054 ↩

  6. Associated Press. (2024, July 26). Wood pellets production boomed to feed EU demand. It's come at a cost for Black people in the South. AP News. https://apnews.com/article/wood-pellets-biomass-climate-environmental-justice-biden-cd9a3de5f55d5acf495986fed8ddc778 ↩ ↩2 ↩3

  7. US Industrial Pellet Association. (2025). USIPA Membership Guide. https://theusipa.org/membership-guide ↩

  8. Sahoo, K., Bilek, E., Bergman, R., & Mani, S. (2019). Techno-economic analysis of producing solid biofuels and biochar from forest residues using portable systems. Applied Energy, 235, 578-590. https://doi.org/10.1016/j.apenergy.2018.10.076 ↩