The transport sector currently accounts for over 30% of the total primary energy consumption in the European Union (EU) and is 98% dependent on fossil fuels. The EU is a net crude oil importer, with an increasing oil dependency ratio (over 0% in 2007, Eurostat). One of the most important energy targets for the EU is thus the reduction of oil use and dependency. A recent EU Directive (2009/28/EC) establishes a 20% target share of renewable energy in primary energy consumption by 2020, with a 10% share of energy from renewable sources for transport. In turn, another EU Directive (2003/96/EC) allows the Member States to have exemptions from or reductions in excise duties so as to promote biofuels and, consequently, there is a growing interest in biofuels in Europe.
In this context, biodiesel constitutes a renewable fuel that is almost compatible with profitable diesel engines and has clear environmental benefits relative to diesel fuel. Biodiesel consists of a mixture of fatty acid methyl esters (FAMEs) obtained from renewable resources, such as vegetable oils and animal fats, by trans esterification with methanol in the presence of an acid or basic catalyst. It can be synthesized from a range of feedstock, but refined vegetable oils (such as soybean, rapeseed, palm and others) are currently the primary industrial feedstock (first generation biodiesel). The relatively high and unstable prices of these food grade raw materials, which are linked to the transfer of land from food to energy crops and also to doubts as to whether biofuels are as environmentally friendly as originally thought, constitute a great obstacle for biodiesel production and commercialization. Likewise, EU Directive (2009/30/CE) establishes, with effect from 1 January 2017, that the greenhouse emission savings from the use of biofuels taken into account for the purpose of 10% shall be at least of 50% (being this value increased up to 60% from 1 January 2018). Annex IV of this Directive indicates that these greenhouse emission savings are in general lower than 50% for the production of biodiesel using conventional edible oils as feedstock, but in contrast, this value is increased to over 80% using waste vegetable or animal oils.
Therefore, it is of main importance to study alternative non-edible waste feedstocks for the sustainable production of second generation biodiesel. Non-edible oils like those derived from jatropha, pongamia, jojoba, and so on are promising feedstock particularly in developing countries where edible oils are in short supply. 1,2 Likewise, waste cooking oil (WCO) is considered an economic and increasingly available resource for biodiesel production. 3,4 The estimated total waste cooking oil capacity in the EU is 700 000–1 000 000 t/year. Although part of the WCO produced is collected for adequate disposal or treatment, most of the domestic WCO is usually discharged into the environment and is the origin of several problems in wastewater depuration systems. The valorization of this used oil through the production of biodiesel contributes to increasing its recovery rate from the waste cycle.
Low-grade or waste animal fats can also be used as raw materials for the production of biodiesel. The production of biodiesel from microalgae, as well as from oleaginous microorganisms, is already considered as interesting feedstock for biodiesel production. Other potential low-cost environmentally friendly sources of biodiesel have been reported. For instance, the solvent extraction of waste coffee grounds yields 10–15% oil depending on the coffee species, which makes this waste a potential candidate for raw material for biodiesel production.
Municipal sewage sludge (MSS) is also gaining attention as a lipid feedstock for biodiesel production owing to its significant concentration of lipids. MSS is plentiful all around the world and its management already poses formidable environmental problems. However, there are still numerous challenges faced in biodiesel production from waste sludge.
Nevertheless, most technologies for the production of biodiesel using low quality feedstock is still in their infancy and research efforts are needed in the future. But the development of these technologies in the future will lead to important benefits for the biodiesel industry: less environmental impact, safer processes, higher glycerol purity, reduction of land use and noninterference in the food chain (Schematic flow chart).
Advances in Biodiesel Production-Schematic flow chart