Science

Biofuels: On the road to carbon-neutrality

With the recent changes from E5 to E10 petrol, biofuels are once again back in the limelight. But what is the benefit of biofuels, and what are the challenges associated with their implementation?

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Image Credit: Dennis Schroeder / NREL

Firstly, what even are biofuels? Biofuels represent mankind’s effort to replace the black elixir that powers our modern world. As the name suggests, biofuels are petroleum replacements derived from biological sources.

However, with biofuels currently accounting for only 3 percent of the transport sector's fuel usage, petroleum fuels dominate. The vast majority of internal combustion engines use hydrocarbon-based fuels derived from crude oil: petrol consists primarily of carbon chains between 4 and 12 atoms long, and diesel from chains 8 to 21 atoms long. Being refined from crude oil, petrol and diesel represent a finite resource that will, in time, run out.

First generation biofuels are those which are manufactured from crops, predominantly corn, sugar cane, and soy, purpose-grown for the fuel industry. After harvest, the crops are processed into petroleum analogues through different processes depending on the desired final product: fermentation of starch rich crops makes bioethanol; transesterification converts the fatty acids found in vegetable oils into biodiesel. Second generation biofuels are chemically similar to first generation, but made from biological waste and residues. Both are typically used in a blend alongside crude oil derived fuels, with a multitude of associated benefits.

Third generation biofuels are the newest, manufactured from a diverse range of renewable feedstocks, from wood, to manure, to the same residues as second generation feedstocks. These more advanced fuels differ from their counterparts in that they are for the most part chemically identical to the fossil fuels they replace. Whilst they can be used in a blend alongside petroleum fuels, their most exciting property is that they have the capability to be used as drop-in replacements, with no need for any form of vehicle modification.

Perhaps the most commonly cited benefit of biofuels is their potential carbon-neutrality; the carbon emitted from their production and combustion is offset by the carbon captured from the atmosphere during the growth of the feedstock. One might assume that all biofuels are carbon-neutral, but this is not always the case, as we’ll see later.

As global demand for fuel increases, petroleum reserves are irreversibly depleted; biofuels, on the other hand, can be produced on demand. Not only does this reduce our dependence on an increasingly scarce resource but it also allows for a greater level of autonomy; currently, the majority of countries have to import oil due to demand outweighing their own supplies. With the increase of biofuel production, this reliance could be lessened as countries become capable of producing their own sustainable fuel source.

Another potentially surprising benefit of biofuels is the performance benefit that some bring. Ethanol, for example, can increase the power of an engine, be that when used neat or simply as an additive in petrol. The oxygen contained in ethanol ensures it burns more completely and releases more energy, with the added benefit of reducing the amount of harmful byproducts produced. Ethanol also keeps your engine cleaner as its strong solvent properties reduce the buildup of carbon deposits in the ports and combustion chamber.

Despite the many benefits of biofuels, they also present some unique challenges. In 2020, only 7 percent of biofuels globally are produced from wastes and residues, with the rest coming from purpose-grown feedstock. From 2020-2021, 35 percent of corn grown in the US was used as a feedstock for ethanol production, and in 2018 it was found that 13 percent of global cropland is used for biofuels and textiles. With a limited amount of fertile land, it is clear that a more efficient system is needed. For biofuels to become a sustainable alternative to fossil fuels, investment into the transportation and processing of bio-waste is required.

As mentioned previously, not all biofuels are carbon-neutral (some can even be carbon-positive), and whilst they may still remain less polluting than their petroleum-based equivalents, the processing required to generate biofuels from raw materials and the increase in agricultural land negates the presumed benefit of crop-derived fuels. Corn, for example, requires fermentation (similar to that used in the alcohol industry) which in turn requires heat, to be converted from its raw state into a usable ethanol fuel. Biodiesel, which can serve as a direct replacement to conventional diesel in some cases, can be carbon-positive due to the large swathes of land that are cleared to make way for the soybean and palm oil trees required to produce it. Water demand for many biofuel crops is higher than that of traditional crops, creating further environmental problems.

Energy density is another concern; pure ethanol contains just 65 percent of the energy in an equivalent volume of petrol, and biodiesel fares slightly better at around 85 percent. Although this results in a miles per gallon drop of around 1-2 percent when used in a blend, it does raise questions as to the mileage limitations that may occur when biofuels are used on their own. The cold is an additional hurdle for biofuels to jump, as they display greater changes in viscosity as temperature changes when compared against the fuel they are replacing. Wax formed within the oil at these lower temperatures can lead to clogs within the internals of the engines fuel distribution system, requiring the inclusion of specialised additives to stabilise it at these lower temperatures.

Without a doubt, biofuel usage must be increased in order to meet emission and climate targets, but are they an answer to a problem that should be mitigated completely? Combustion engines are inherently inefficient and should be replaced entirely by propulsion methods that do not waste as much energy. Modern electric cars can convert upwards of 85 percent of their stored energy into motion; for context, even the most efficient petrol cars only manage a maximum of 40 percent. However this serves as a poignant reminder of the privileged position we hold to be able to choose between an electric and combustion powered car. With developing nations producing up to 130 percent of the emissions of developed ones, their reliance on and environmental impact from using fossil fuels can certainly be lessened by increasing biofuel usage.

In summary, there are many complex factors that have to be considered when planning for a future powered by biofuels. Whilst they have many clear benefits over petroleum-based fuels, the problems caused through competition for arable land and overall reduction in biodiversity cannot be ignored.

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