There is no doubt that the energy potential of biomass on earth is enormous, estimations from 2014 suggests that the global annual production of biomass is approximately 100 billion tons per year (Wang et al., 2017). A promising biomass with huge theoretical potential is lignocellulosic biomass which comprises mainly forestry and agricultural waste which is nonedible second-generation biomass. An exceptional energy source in the form of lignocellulosic biomass are energy crops, which can be harvested on annual basis making their carbon cycle short. Energy crops can be processed by thermochemical conversion to further produce biofuels which can be used as fuel for transportation. In 2017 only 7.4 % of the energy utilized by the transportation sector in the EU was from renewable sources and as of 2050 the goal is to achieve net zero greenhouse gas emissions. The International Energy Agency (IEA) believes that biofuels can replace up to 27 % of the world’s transportation fuels by 2050 (International Energy Agency, 2011). However, to utilize biomass as transportation fuel, thermochemical processes are applied to extract bio oil which further can be upgraded to second generation biofuels (Mortensen et al., 2011). The most extensively used process for this is fast/flash pyrolysis where the biomass is heated at high heating rates with short residence times which favours a high liquid production. The produced bio oil has a higher heating value (HHV) of approximately 40 % of crude oil. Hence it requires complicated methods for upgrading before it can serve as an alternative for crude oil. The three major components of lignocellulosic biomass are cellulose, hemicellulose and lignin. Depending on the type of biomass the distribution of these main components varies quite a lot. Also, the structure of the components has a high correlation with how it reacts during pyrolysis hence also influencing the outcome of the end products from pyrolysis (Wang et al., 2017). Apart from the liquid product (bio oil), pyrolysis also produces char (solid) and a gaseous product, in a bio oil producing facility the char and gas can be combusted to supply the required heat for the pyrolysis process.
Figure 1 pathway from biomass to upgraded biofuels (Wang et al., 2017).
Processing biomass into biofuels is energy demanding but has a huge potential considering that end products that can be obtained. Further it can be classified as a net zero producer of CO2 emissions if annual crops are utilized. That this would entirely replace conventional fuels in the transportation sector is a longshot. If crude oil is to be replaced the production of bio oil needs technical advancements, more understanding needs to be obtained regarding the reaction mechanisms of both pyrolysis and the upgrading methods to able to design and operate large scale production of bio fuels. Positively, biomass pyrolysis has gotten a lot more attention, 800 % more journal papers was published on the subject in 2016 compared to 2005 (Wang et al., 2017). Hopefully increasing research will lead to a future breakthrough and large-scale biofuel production can present in a large reduction of CO2 emission from the transportation sector.
International Energy Agency. (2011). Technology Roadmap (p. 56). https://www.ieabioenergy.com/wp-content/uploads/2013/10/IEA-Biofuel-Roadmap.pdf
Mortensen, P. M., Grunwaldt, J.-D., Jensen, P. A., Knudsen, K. G., & Jensen, A. D. (2011). A review of catalytic upgrading of bio-oil to engine fuels. Applied Catalysis A: General, 407(1), 1–19. https://doi.org/10.1016/j.apcata.2011.08.046
Wang, S., Dai, G., Yang, H., & Luo, Z. (2017). Lignocellulosic biomass pyrolysis mechanism: A state-of-the-art review. Progress in Energy and Combustion Science, 62, 33–86. https://doi.org/10.1016/j.pecs.2017.05.004