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Lignocellulosic biomass is the most abundant form of biomass on Earth, composed of cellulose, hemicellulose, and lignin

Summary​

Lignocellulosic feedstock is the most abundant form of biomass on Earth and serves as a key resource for second generation biofuels. Composed primarily of lignin, cellulose, and hemicellulose, this biomass requires extensive pretreatment to access fermentable sugars so they can be converted into ethanol. The three main sources of lignocellulosic biomass are agricultural residues like corn stover, forest wastes and residues, and dedicated energy crops such as switchgrass and willow. While agricultural residues are considered desirable due to their waste status, their removal can impact soil health. Forest residues remain contentious, and energy crops face scrutiny over land competition concerns.

Lignocellulosic feedstock has been proposed as a potential feedstock for second generation biofuels due to its abundance, as well as its potential to valorize waste streams. Lignocellulosic biomass (sometimes called cellulosic biomass) is the most abundant form of biomass on earth 1, and is primarily comprised of lignin (26 – 31%), cellulose (41 – 46%), and hemicellulose (25 – 32%) 2. Cellulose and hemicellulose are polysaccharides, which are essentially long chains of sugars (glucose in the case of cellulose and xylose in the case of hemicellulose) that can be broken down enzymatically, after which they can be fermented into alcohol 3. Pentose, the generalized name for 5 carbon sugars like xylose, cannot be fermented by naturally occurring yeasts and can sometimes form molecules that inhibit the hydrolysis of cellulose, meaning their presence and be undesirable 4. Lignin is a complex biopolymer that acts as the cement that holds the remaining cellulosic biomass together and provides rigidity and microbial protection to the biomass structure 5. The existence of lignin in the biomass structure is one of the main challenges to processing lignocellulosic biomass, often requiring expensive and energy-intensive pretreatments to access the cellulose and hemicellulose held firm by the lignin.

The three most common forms of lignocellulosic feedstock are:

Pretreatment​

In order for the desirable elements of lignocellulosic biomass to be accessed for the biochemical conversion of biomass into ethanol, they must first undergo some degree of pretreatment. As already mentioned, the high content of lignin and presence of pentose in the biomass structure is one of the key challenges for utilizing lignocellulosic biomass, and the primary goal of pretreatment is to break down the lignin (and remove the pentose if desired) so that the cellulose can be accessed. In the pretreatment process, the hemicellulose is usually partially decomposed into its main sugar building block called xylose after which it can be easily separated or fermented into ethanol 6.

There are a number of different pretreatment methods that can be used, including 7:

  • Acid hydrolysis
  • Lime pretreatment
  • Ammonia fiber explosion
  • Liquid hot water
  • Steam explosion
  • Organic solvent
  • Alkaline wet oxidation
  • Ionic liquid

Each pretreatment method has its own set of advantages and disadvantages, which can be chosen based on the feedstock and the bioconversion pathways after pretreatment 6. Pretreatments that use acids or solvents may have a high level of material recovery by freeing up the most cellulose for the fermentation stage, but may require additional cleaning steps and high water usage to prepare the biomass. Pretreatments that rely on pressure and temperature may result in a material that is easier to handle at the cost of high energy usage, possible increasing the overall emission intensity of the process.

Sources​

Footnotes​

  1. Liu, Y., Cruz-Morales, P., Zargar, A., Belcher, M. S., Pang, B., Englund, E., Dan, Q., Yin, K., & Keasling, J. D. (2021). Biofuels for a sustainable future. Cell, 184(6), 1636–1647. https://doi.org/10.1016/j.cell.2021.01.052 ↩

  2. Malode, S. J., Prabhu, K. K., Mascarenhas, R. J., Shetti, N. P., & Aminabhavi, T. M. (2021). Recent advances and viability in biofuel production. Energy Conversion and Management: X, 10, 100070. https://doi.org/10.1016/j.ecmx.2020.100070 ↩

  3. Timilsina, G. R., & Shrestha, A. (2011). How much hope should we have for biofuels? Energy, 36(4), 2055–2069. https://doi.org/10.1016/j.energy.2010.08.023 ↩

  4. International Renewable Energy Agency. (2016). Innovation outlook: Advanced liquid biofuels. IRENA. https://www.irena.org/publications/2016/Oct/Innovation-Outlook-Advanced-Liquid-Biofuels ↩

  5. Callegari, A., Bolognesi, S., Cecconet, D., & Capodaglio, A. G. (2019). Production technologies, current role, and future prospects of biofuels feedstocks: A state-of-the-art review. Critical Reviews in Environmental Science and Technology, 50(4), 384–436. https://doi.org/10.1080/10643389.2019.1629801 ↩

  6. 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 ↩ ↩2

  7. Murali, G., & Shastri, Y. (2019). Life-cycle assessment-based comparison of different lignocellulosic ethanol production routes. Biofuels, 13(2), 237–247. https://doi.org/10.1080/17597269.2019.1670465 ↩