The agro-food industry generates large amounts of waste that contribute to environmental contamination. Animal fat waste constitutes some of the most relevant waste and the treatment of such waste is quite costly because environmental regulations are quite strict. Part of such costs might be reduced through the generation of bioenergy. Biodiesel constitutes a valid renewable source of energy because it is biodegradable, non-toxic and has a good combustion emission profile and can be blended up to 20% with fossil diesel for its use in many countries. Furthermore, up to 70% of the total cost of biodiesel majorly depends on the cost of the raw materials used, which can be reduced using animal fat waste because they are cheaper than vegetable oil waste. In fact, 6% of total feedstock corresponded to animal fat in 2019. Transesterification with alkaline catalysis is still preferred at industrial plants producing biodiesel. Recent developments in heterogeneous catalysts that can be easily recovered, regenerated and reused, as well as immobilized lipases with increased stability and resistance to alcohol denaturation, are promising for future industrial use. This manuscript reviews the available processes and recent advances for biodiesel generation from animal fat waste.

Biodiesel constitutes an attractive source of energy because it is renewable, biodegradable, and non-polluting. Up to 20% biodiesel can be blended with fossil diesel and is being produced and used in many countries. Animal fat waste represents nearly 6% of total feedstock used to produce biodiesel through alkaline catalysis transesterification after its pretreatment. Lipase transesterification has some advantages such as the need of mild conditions, absence of pretreatment, no soap formation, simple downstream purification process and generation of high quality biodiesel. A few companies are using liquid lipase formulations and, in some cases, immobilized lipases for industrial biodiesel production, but the efficiency of the process can be further improved. Recent developments on immobilization support materials such as nanoparticles and magnetic nanomaterials have demonstrated high efficiency and potential for industrial applications. This manuscript reviews the latest advances on lipase transesterification and key operational variables for an efficient biodiesel production from animal fat waste.

Gas exhaust emissions in vehicles are increasingly restrictive in EU and USA. Diesel engines are particularly affected by limitation in hydrocarbons and NOx concentrations. This work presents a screening of working electrode materials to develop a potentiometric sensor, with the most promising material to detect being C₂H₄ at 550 °C. The device consists of a dense 8YSZ (8 mol% Y₂O₃ stabilized ZrO₂) disk as oxide-ion conducting electrolyte, whereas platinum is screen-printed in the back face as reference electrode. As working electrode, several materials such as Fe_0.7Cr_1.3O₃, ZnCr₂O₄, Fe₂NiO₄, La_0.8Sr_0.2CrO_3−δ (LSC), La_0.8Sr_0.2MnO₃ (LSM), and NiO+5%wt Au were tested to detect C₂H₄. Sensor voltage was measured for several concentrations of C₂H₄ and CO as these are two of the major oxidizable compounds in a diesel exhaust gas. Fe_0.7Cr_1.3O₃ was selected as the most promising material because of its response to C₂H₄ and CO. Not only is the response to the individual analytes important, but the C₂H₄ cross-sensitivity toward CO is also important. Fe_0.7Cr_1.3O₃ showed a good performance to C₂H₄, with low cross-sensitivity to CO. In addition, when 0.16 ppm of phenanthrene is added, the sensor still has a slightly better response to C₂H₄ than to CO. Nevertheless, the sensor exposure to high concentrations (>85 ppm) of polycyclic aromatic hydrocarbons led to signal saturation. On the other hand, the operation in wet conditions induces lower sensor sensitivity to C₂H₄ and higher cross-sensitivity toward CO increase, i.e., the sensor response becomes similar for C₂H₄ and CO.

Ultra-short-pulse laser radiation has been used to modulate the electrical conductivity of ZnO thin films deposited by spatial atomic layer deposition. Resistance time evolution exhibits high sensitivity to oxygen concentration in the environment.