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Advanced biofuels have been developed to address the limitations of first generation biofuels but have so far failed to achieve commercial success

Summary​

Advanced biofuels were developed to address the limitations of first-generation biofuels, particularly their impact on food systems and questionable environmental benefits. These biofuels are categorized into second, third, and fourth generations, each representing theoretical progressions in technology rather than commercial realities. Second-generation biofuels use non-food lignocellulosic biomass but face energy-intensive processing challenges. Third-generation biofuels utilize microalgae for feedstock, but encounter struggles with cultivation costs and contamination. Fourth-generation biofuels employ genetically modified microalgae to enhance CO₂ capture and yields, though they raise concerns about genetically modified organisms escaping to natural populations. As of 2025, the biofuel industry remains dominated by first-generation biofuels despite these advancements.

Advanced biofuels were first envisioned as a way to address the perceived failures of the first generation of biofuels, primarily with regard to their influence on global food shortages and the relatively insignificant environmental benefits. The term 'advanced biofuels' is not well defined, with a number of legal definitions existing in the United States and Europe that are designed to provide guidelines for subsidies and tax incentives. The International Renewable Energy Agency (IRENA) highlights a number of categories that may result in a biofuel being classified as an 'advanced biofuel' 1. These categories include:

  • Feedstock that is not food crops
  • GHG emissions savings
  • Technological maturity
  • Final product type

This analysis will focus on a broader definition of advanced biofuels to include all biofuels that are not first generation biofuels, i.e., biofuels that are not made from food crops. Under this definition, advanced biofuels can be broken down into the second, third, and fourth generation of biofuels. This naming convention can be somewhat misleading, as the use of the term 'generation' often implies that there has been some form of progression or transition to reach the next generation. This is not the case with biofuels, as each generation is more so a theoretical progression of the preceding generation to address its shortcomings. In regards to commercial biofuels, as of 2025, the industry has not moved past the first generation.

Second generation biofuels​

Second generation biofuels were developed as a response to many of the controversies that arose from the feedstock needs of first generation biofuels, specifically land usage and food scarcity issues. To achieve this, second generation biofuels focused on lignocellulosic matter as their primary feedstock, which is biomass composed primarily of cellulose, hemicellulose, and lignin, which can be sourced from agricultural and forest residues, and energy crops. By utilizing feedstocks that are non-edible, in theory there is less pressure on food systems. There are a range of pathways for second generation biofuels including the thermochemical pathway, biochemical pathway or a hybrid of the two 2. The thermochemical pathway utilizes pyrolysis and gasification to convert lignocellulosic biomass into biogas or producer gas which can then be converted to higher value products through a Fischer-Tropsch process 3. The biochemical pathway utilizes a series of chemical, biological and sometimes mechanical processes to convert the lignocellulosic biomass into ethanol 3. Second generation bioethanol have a lot of similarities to first generation bioethanol, except the second generation requires more pretreatment steps to access the sugars or starches before then can be fermented, whereas the sugars and starches from first generation feedstocks like corn and sugar cane can be directly accessed and fermented. Second generation biofuels can also utilize non-edible oils as a feedstock to create biodiesel via transesterification 3. A key downside of second generation biofuels is they require a number of energy intensive and costly steps to break down the lignocellulosic biomass into a form where the sugars can be accessed and fermented 2. Additionally, concerns relating to indirect land use change (ILUC), soil organic carbon, and intensive agricultural farming practices are still present.

Third generation biofuels​

Like second generation biofuels, third generation biofuels were developed to address the issues of the two preceding generations mainly relating to feedstock. This search for alternative feedstocks led to the development of third generation biofuels that utilize microalgae organisms and sometimes macroalgae as their feedstock. Algae are attractive as a feedstock due to their high lipid productivity (meaning they are oil rich) and their ability to grow and be harvested quickly in around 5 to 6 days 2. Algae are highly effective at photosynthesis with rates 2x to 4x higher than that of terrestrial plants 2, and they naturally combine the energy capture and fuel production step within their cells 4. Cultivation of microalgae can occur in specialized tanks known as algal bioreactors that can be located anywhere and have their temperature optimized for algae growth, or in open ponds that lack the optimization control but are generally cheaper to operate 5. Estimates for theoretical microalgae oil production are very high, reaching up to 26,400 gallons (100,000 liters) per hectare per year compared to 260 - 1,900 gallons (1,000 – 6,000 liters) per hectare per year for palm and sunflower oil 5. Nonetheless, attempts made at cultivating microalgae on a large scale have largely proven to be difficult and costly, with algal bioreactors being very expensive and open ponds easily susceptible to contamination 5. Additionally, the processing of microalgae into a drop-in biofuel is still costly and energy intensive, with the majority of successfully cultivated microalgae being diverted to food and feed uses 4.

Fourth generation biofuels​

Fourth generation biofuels build further upon third generation biofuels by utilizing genetically modified microalgae. The genetic modification has two goals, the first being to increase the amount of CO₂ that is captured by the algae and the second being to increase the biofuel yields by boosting growth rates and adaptation to suboptimal cultivation conditions 2. An example of a genetic modification for microalgae is widening the spectrum of light that can be absorbed by the antennae system to allow for more efficient photosynthesis 5. The largest concern in relation to fourth generation biofuels are the serious dangers of genetically modified organisms escaping into nature when it can mix its genes into the natural gene pool 2 5. Because of this, the majority of research into fourth generation biofuels has focused on the possible effects of genetically modified microalgae on the environment 2.

Sources​

Footnotes​

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

  2. Mat Aron, N. S., Khoo, K. S., Chew, K. W., Show, P. L., Chen, W.-H., & Nguyen, T. H. P. (2020). Sustainability of the four generations of biofuels – A review. International Journal of Energy Research, 44(13), 9266–9282. https://doi.org/10.1002/er.5557 ↩ ↩2 ↩3 ↩4 ↩5 ↩6 ↩7

  3. 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 ↩3

  4. 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

  5. Cavelius, P., Engelhart-Straub, S., Mehlmer, N., Lercher, J., Awad, D., & Brück, T. (2023). The potential of biofuels from first to fourth generation. PLoS Biology, 21(3), e3002063. https://doi.org/10.1371/journal.pbio.3002063 ↩ ↩2 ↩3 ↩4 ↩5