An Unlikely Contender on the Path to Renewable Energy

By Erin Kang
August 12, 2020 · 4 minute read



Environmental Science

What if I told you that oranges could be the key to renewable energy in the future? In this day and age, our dependence on fossil fuels has led to severe consequences, including global warming. Scientists around the world are racing to find an environmentally friendly solution. Oranges, more specifically orange peels, are slowly, but surely, climbing to the top.

The peels of oranges are often thrown away in waste. Approximately 3.8 million tons of orange peels per year result in waste. In terms of disposal, burning the waste directly contributes to greenhouse gas emissions, which makes it a consequential environmental liability. However, the peel is arguably one of the essential parts of an orange. Limonene is a natural cyclic monoterpene, more commonly referred to as d-limonene, which is its main chemical form. Limonene is an incredibly versatile agent that has a diverse range of applications, varying from its use in cleaning supplies to the possible use of its therapeutic properties in anticancer drugs. D-limonene is found in the peels of citrus fruits and is a renewable chemical that does not contain any environmentally harmful additives. It also shares several chemical similarities with the fossil fuels that we use to power our world every day. Furthermore, d-limonene is easily combustible and has a low freezing point and high energy density. Therefore, making it an ideal candidate to convert into biofuel.

There has been much research done regarding the different ways to convert orange peels into biofuel. There are several ways to extract the oil from the peel, including hydro distillation, steam distillation, and cold press. Steam distillation is when the orange peels are put into a compartment and mixed with water. Then, the substance boils. The vapor mixture of the oil and water passes through a cooling system filled with cold water. The vapor is then condensed to a separate oil and water distillate. The limonene can be extracted using a separatory funnel. However, this method has its disadvantages—one being the less than adequate yield. The expected yield using steam distillation is about 1% of the total mass of peel used. Although it may be a suitable procedure for personal use, it is less efficient in mass production. Another disadvantage could be the extraction of the limonene from the distillate. As the water and oil layers are on top of each other, you need to be careful not to collect any part of the water layer. 

Below is a video of an experiment performed to extract the limonene from orange peels. The video shows a fun way to observe limonene and its flammable property in use. 

Researchers at the University of York are part of a “zero waste” biorefinery project that will use low-temperature, high-intensity microwaves to extract the oil from the peels of oranges (Casey, 2012). The researchers placed orange peels and an organic solvent into the microwave and heated it for 30 minutes. During those 30 minutes, the water started to boil, rupturing the cells, and allowing the limonene to leech out of the peel. Microwave-assisted extraction was shown to be the more efficient method of limonene extraction than some of the more conventional methods. Employing microwave-assisted extraction leads to a significantly shorter time needed to extract the limonene, a greater yield of d-limonene overall, higher quality d-limonene extracted, and this method is less energy-intensive (Hutchby, 2015).

Another unconventional method of using oranges as a source of biofuel stems from the research done by Henry Daniell from the University of Central Florida. His technique utilizes plant-derived enzymes to break down the peels of oranges into sugar. The sugar is then fermented into ethanol (Ford, 2010). Although the ethanol created using this technique would still produce greenhouse gas emissions, the amount of greenhouse gas emissions is much lower than that of ethanol from corn or petroleum. The pectinase enzyme is an enzyme that breaks down pectin. Pectin is a polysaccharide that is found in the primary cell wall of many fruits. The use of pectinase enzymes resulted in high levels of monomeric sugars. This enzyme can be produced from a tobacco plant, instead of synthetically made. By doing so, the cost of production decreases drastically, making it extremely efficient.

Limonene is also an alternative to bisphenol-A, a dangerous component that is used in polycarbonates. Although many plastics are “BPA-free,” the replacement, bisphenol S, may be just as harmful. By incorporating limonene, researchers have been able to successfully create a more environmentally friendly polymer, while simultaneously improving the polymer’s thermal properties (Kleij, 2017).

Places such as Florida and California, where there is a tremendous waste of orange peels, will most likely take the lead in this initiative. As much as research has advanced and new methods have been developed, the path to a greener world is still long. However, there is no doubt that orange peels will be an essential part of our future. Whether through biofuel or renewable plastics, orange peels can be an efficient part of a global initiative to power our world. 


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