Biodiesel at the Crossroads: A Critical Review

Abstract

The delay in the energy transition, focused in the replacement of fossil diesel with biodiesel, is mainly caused by the need of reducing the costs associated to the transesterification reaction of vegetable oils with methanol. This reaction, on an industrial scale, presents several problems associated with the glycerol generated during the process. The costs to eliminate this glycerol have to be added to the implicit cost of using seed oil as raw material. Recently, several alternative methods to convert vegetable oils into high quality diesel fuels, which avoid the glycerol generation, are being under development, such as Gliperol, DMC-Biod, or Ecodiesel. Besides, there are renewable diesel fuels known as “green diesel”, obtained by several catalytic processes (cracking or pyrolysis, hydrodeoxygenation and hydrotreating) of vegetable oils and which exhibit a lot of similarities with fossil fuels. Likewise, it has also been addressed as a novel strategy, the use of straight vegetable oils in blends with various plant-based sources such as alcohols, vegetable oils, and several organic compounds that are renewable and biodegradable. These plant-based sources are capable of achieving the effective reduction of the viscosity of the blends, allowing their use in combustion ignition engines. The aim of this review is to evaluate the real possibilities that conventional biodiesel has in order to success as the main biofuel for the energy transition, as well as the use of alternative biofuels that can take part in the energy transition in a successful way.

Diethyl Ether as an Oxygenated Additive for Fossil Diesel/Vegetable Oil Blends: Evaluation of Performance and Emission Quality of Triple Blends on a Diesel Engine

“…In this regard, the transformation of vegetable oils into high-quality diesel fuels, avoiding the glycerol formation, has been deeply investigated [11]. These biodiesel-like biofuels, such as Gliperol [12], DMC-BioD [13,14] or Ecodiesel [15,16], avoid the generation of residues or by-products, integrating glycerol in the reaction products.…”

Recent Advances in Glycerol Catalytic Valorization: A Review

“…That is the reason why the rest of the options are more popular in research. Nevertheless, new trends are currently being considered for the direct use of crude or residual glycerol, such as its use as an additive to produce biohydrogen by the anaerobic biodigestion of cassava wastewater [39]; • Preventing its production: In this case, other derivatives of glycerol are produced during transesterification by different means, resulting in interesting compounds, such as glycerol triacetate (Glyperol), glycerol carbonate (DMC-Biod), and monoglyceride (Ecodiesel), which contribute to a better atom yield during the transesterification of fatty acids [40,41]; • Purification: As abovementioned, crude glycerol has low prices, with its purification being expensive for small companies. This fact can explain the existence of the two previous options mentioned, and the research related to glycerol purification to develop a cheaper and more competitive process.…”

Biodiesel Production Using Solid Acid Catalysts Based on Metal Oxides

“…It was reported in various studies that 10%-20% of the total volume of biodiesel produced is composed of glycerol; for every 100 kg of biodiesel produced, 10 kg of glycerol is generated [51]. Glycerol is, thus, a cheap and abundant renewable feedstock, not only because of the huge amount produced, but also due to its low quality, since it is mixed with other products such as methanol, water, and salts [52]. On the other hand, it is a natural, sweet-tasting thick colorless liquid considered a base product, which has diverse applications in various industries such as pharmaceuticals, personal care, and food and beverages, as well as in the production of tobacco, alkyd resins, and polyether polyols; on the other hand, compounds of glycerol are used to preserve lotions, inks, fruits, and lubricants.…”