All around the world, oil has been variously used for cooking purposes for centuries. Its major use, is as a medium for heat transfer when frying and of course as an additive for flavour and texture. There is a big variety of cooking oils derived from plants such as olive, sunflower and rapeseed that are mostly used in Europe, or soybean and palm oil that are widely used in south America and south-east Asia. In addition, butter and lard are considered as animal-based cooking oils. However, cooking oil does not stop being useful after being fried. Used cooking oil is considered a low-cost and renewable feedstock for the production of biodiesel and other biobased products.
Europe and many individual countries have set goals regarding the climate change. Moreover, Sweden aims to be the first country that will become fossil free in the transportation sector by 2030. Thus, renewable fuels have become one of the main pillars to succeed such a goal. Vegetable oil is an already commercial used feedstock for the production of different renewable fuels such as biodiesel or hydrotreated vegetable oil (HVO). However, the use of such an edible feedstock is competing with food industry and poses the global dilemma of the need of feeding humanity versus the exploitation of land for agro-energy. Such a dilemma can be avoided with the use of used cooking oil (UCO) instead. Since the oil has already been used for cooking purposes it is considered a waste, hence the biofuels produced from UCO are as stated “second generation” biofuels.,
During the last decade a significant increase in the usage of UCO has been reported in Europe. Even though rapeseed oil (RO) still remains the dominant biodiesel raw material, its share in the feedstock mix has decreased from 72% in 2008 to 47% in 2016. This is due to the use of recycled cooking oil that has become the second most important feedstock in Europe accounting to 18% in 2016. In Figure 1. a simplified process flow is illustrating the upgrade of waste oils to Hydrotreated Vegetable Oils (HVO) such as kerosene as a renewable jet fuel or green diesel as a renewable fuel used in the transport section. As it can be seen from the figure the UCO needs to be pretreated before the phase of hydrotreatment where hydrogen is added. Based on the feedstock green diesel could be classified as biodiesel, however, based on the processing technology and chemical formula green diesel and biodiesel are different products.,,
Overall, as presented UCO exploitation can involve large reductions in life cycle impacts, cutting the need for virgin vegetable oil for fuel production and promoting a way of waste management for this type of waste. Furthermore, some additional advantages of this biomass resource are that it can be found in abundance and since it is waste that is non-edible anymore it does not longer compete with food. Finally, it is a feedstock that depending on the chosen method can produce a variety of bio-products. Despite UCO seems to be an attractive choice of feedstock, there are some challenges that need to be overcome. Firstly, the supply chain plays a major role in the sustainability of the proposed production schemes, hence life cycle assessment of the feedstock must be done to examine the environmental impacts of the feedstock from its harvest to the collection point of UCO and it is necessary to deploy effective policies and regulated practices to enhance UCO recycling and collection rates, under multistakeholder considerations. Moreover, UCOs have a highly heterogenous nature, depending their origin, that make them having a big variety of different properties (i.e physicochemical, impurities, color, odor), thus the right pre-treatment method must be chosen.