Not so carbon neutral: the story of biofuels

By Ryan Tam – Chemical Engineering Student @ Trinity College, Cambridge

 

Biofuels are a growing class of renewable energy resources that are looking to replace the fossil-fuel-based status quo. Made from organic material, they are presented as a carbon-neutral alternative that can be easily manufactured. The most common biofuels include biogas, bioethanol, and biodiesel, and each of them come with their own unique production methods and uses. However, do they really live up to their claim of being carbon neutral?


The thing that all biofuels have in common is that they are made from organic material. Biogas can be produced by the breakdown of waste material by anaerobic bacteria to produce methane gas, the same gas found in natural gas. These bacteria do not require oxygen in order to break down organic material and can reliably produce methane.


Bioethanol is produced from the fermentation of plant sugars by yeast, and typically corn or cane sugar are used as the starting material. Bioethanol is often combined with petrodiesel in order to produce E10 petrol, which produces less particulate matter when combusted as compared to petrol. Biodiesel is produced from plant oils or animal fat, which reacts with methanol in a process known as transesterification. The ester links in the fat molecules are broken, and the fatty acid molecules form new ester functional groups with the methanol molecules. The final biodiesel product consists of long-chain esters; this is what biodiesel is made of.


All of these biofuels are entirely renewable, that is the organic material used to produce these fuels can be replaced at the same rate at which the fuel is consumed. Fuel can continue to be consumed as long as there are sufficient starting materials. Furthermore, even though the combustion of these biofuels releases carbon dioxide into the atmosphere, this is offset by the amount of carbon absorbed from the atmosphere when the crops are grown. This seems to suggest that biofuels are a clean and reliable energy resource, but is this really the case?


However, when crops are harvested, they also release carbon. Large fields must be ploughed and cleared using machinery that emits carbon into the atmosphere. The transportation of the raw material and the processed fuel leads to further carbon emissions, as do the machines that produce the fuels themselves.


Another factor to consider is that the amount of energy released per kilogram of biofuel is smaller than the amount of energy released per kilogram of natural gas or petrol. The ethanol and ester molecules contain oxygen atoms and are therefore partially oxidised, so when combusted, a smaller amount of energy is released. The mixture of gases obtained during the production of biogas also contains a smaller proportion of methane gas, as compared to natural gas. Biofuels are less energy-efficient since a larger amount is required in order to obtain the same amount of energy, making it harder for storing and transporting biofuels.


In particular, biodiesel is difficult to store and combust using common petrol engines in typical vehicles. Biodiesel is quite viscous, and in colder climates, it becomes very resistant to flow, and may even freeze. This may clog up fuel lines and prevent engines from starting. Furthermore, biodiesel and ethanol do not vapourise as readily as petrol and diesel, making it harder to burn the fuel in a combustion engine.


While alternative fuels are often heralded to be the solution to decarbonising our energy resources, it is important to also remember that carbon is emitted every single step of the way: from planting and harvesting the crop, to processing and combusting the fuel. Additionally, other logistic challenges arising from the chemical properties of these organic molecules make it all the more difficult to transition from a fossil-fuel driven society to one entirely dependent on biofuels.


Further Reading:

1. https://www.europeanbiogas.eu/


2. https://www.olleco.co.uk/sustainability/biodiesel


3. https://grist.org/energy/biofuels-are-a-controversial-climate-solution-could-they-still-help-save-the-planet-negative-emissions-direct-air-capture/