Oil-based feedstock from non-food oil sources such as waste oils and non-edible seed oils can be converted into biodiesel through the process of transesterification

Summary

Oil-based feedstocks provide an alternative pathway for second generation biofuels, primarily producing biodiesel through transesterification processes. These feedstocks include non-food sources like jatropha, tobacco seeds, waste cooking oils, animal fats, and sea mango. Animal fats are particularly valued for their beneficial properties and waste valorization potential. Jatropha was once promoted as a 'wonder crop' for marginal lands but faced challenges with lower-than-expected yields, leading to widespread cultivation abandonment.

---

Oil-based feedstocks are an alternative to lignocellulosic feedstock in the production of 2nd generation biofuels that can be used to create biodiesel, also known as oil (m)ethyl esters. The actual process of refining the oil into a usable fuel is not too dissimilar to first generation biofuels that utilize vegetable oils from edible plants such as soybean oil and palm oil. These oils can then be upgraded into a form of sustainable aviation fuel (SAF) known as Hydroprocessed Esters and Fatty Acids Synthetic Paraffinic Kerosene (HEFA-SPK). Examples of second generation feedstocks for biofuels can include ¹:

• Jatropha
• Tobacco seeds
• Waste cooking oils
• Beef tallow
• Sea mango

Animal fats are considered the ideal second generation feedstock as they have beneficial properties such as non-corrosiveness and higher octane values, whilst also valorizing a potential waste product ¹. However, animal fats tend to be harder to process via transesterification. The most studied and tested feedstock for second generation biodiesel is from the energy crop Jatropha Curcas, which was lauded as a 'wonder crop' due to its ability to grow on marginal land and the ability to increase soil organic carbon ². Jatropha is 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%) ³. The seeds of the Jatropha plant are considered toxic when consumed, meaning that it technically meets the classification of a non-food crop ². In the late 2000s, cultivation of Jatropha was heavily promoted mainly in developing economies such as India and countries within Africa including Ghana and Mozambique, with millions of hectares of Jatropha planned to be cultivated ^{4 5}. However, although Jatropha has the ability to grow on marginal land with low water requirements, the actual oil yields from this form of cultivation proved to be significantly lower than the promised high yields ⁴. Currently, almost all Jatropha cultivations have been abandoned or transitioned into other crop uses ⁵.

Transesterification

Transesterification is the simplest and lowest cost method of converting second generation biofuel oils into usable fuels ³. The goal of transesterification is to convert the oil feedstock into a form that has a lower viscosity ⁶. Transesterification is the process of breaking down glycerides (lipids with an attached glycerol molecule) into fatty acid methyl esters (known as FAME or biodiesel) and glycerol by reacting the glycerides with alcohol (can be ethanol, methanol, etc.) ³. The process can be done either with or without a catalyst. In the catalytic process, a catalyst (usually potassium or sodium alkoxides) is used to assist the solubility of the alcohol in the presence of oil to break down the glycerides in the following multi-step process ^{3 6}:

$$\begin{align*} \mathrm{Triglycerides + Alcohol \xleftrightarrow{\text{CATALYST}} Diglycerides + Biodiesel} \end{align*}$$ $$\begin{align*} \mathrm{Diglycerides + Alcohol \xleftrightarrow{\text{CATALYST}} Monoglycerides + Biodiesel} \end{align*}$$ $$\begin{align*} \mathrm{Monoglycerides + Alcohol \xleftrightarrow{\text{CATALYST}} Glycerol + Biodiesel} \end{align*}$$

In the non-catalytic process known as transesterification with supercritical methanol (SCM), the mixture of fats and supercritical methanol (methanol that exhibits both liquid and gas properties) are heated to temperatures and pressures between 483 - 752°F (251–400°C) and 35–60 MPa where the glycerol can be removed from the fat molecules without a catalyst ⁶. The non-catalytic process is desirable due to its low energy usage, rapid reaction time, and no requirement for catalyst handling and removal, however, it does require costly and specialized reactors and high levels of methanol ³.

Sources

Footnotes

1. Alalwan, H. A., Alminshid, A. H., & Aljaafari, H. A. S. (2019). Promising evolution of biofuel generations: Subject review. Renewable Energy Focus, 28, 127–139. https://doi.org/10.1016/j.ref.2018.12.006 ↩ ↩²

2. Jongschaap, R. E. E., Corré, W. J., Bindraban, P. S., & Brandenburg, W. A. (2007). Claims and facts on Jatropha curcas L. : global Jatropha curcas evaluation. breeding and propagation programme. (Report / Plant Research International; No. 158). Plant Research International. https://edepot.wur.nl/41683 ↩ ↩²

3. Atabani, A. E., Silitonga, A. S., Badruddin, I. A., Mahlia, T. M. I., Masjuki, H. H., & Mekhilef, S. (2012). A comprehensive review on biodiesel as an alternative energy resource and its characteristics. Renewable and Sustainable Energy Reviews, 16(4), 2070–2093. https://doi.org/10.1016/j.rser.2012.01.003 ↩ ↩² ↩³ ↩⁴ ↩⁵

4. 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 ↩ ↩²

5. Antwi-Bediako, R., Otsuki, K., Zoomers, A., & Amsalu, A. (2019). Global Investment Failures and Transformations: A Review of Hyped Jatropha Spaces. Sustainability, 11(12), 3371. https://doi.org/10.3390/su11123371 ↩ ↩²

6. Demirbas, A. (2007). Progress and recent trends in biofuels. Progress in Energy and Combustion Science, 33(1), 1–18. https://doi.org/10.1016/j.pecs.2006.06.001 ↩ ↩² ↩³