Though a considerable number of publications about biodiesel can be found in literature, several problems remain unsolved, encompassing economical, social, and technical issues. Thus, the biodiesel industry has come under attack by some environmental associations, and subsidies for biofuel production have been condemned by some governments. Yet, biodiesel may represent a truly competitive alternative to diesel fuel, for which fuel tax exemption and subsidies to energetic crops are needed. Biodiesel must increase its popularity among social movements and governments to constitute a valid alternative of energy source. In this sense, the use of nonedible oils to produce biodiesel is proposed in the present review. Moreover, the compromise of noninterference between land for energetic and food purposes must be addressed. Concerning technical issues, it is important to consider a transesterification optimization, which is missing or incomplete for too many vegetable oils already tested. In most cases, a common recipe to produce biodiesel from any raw material has been adopted, which may not represent the best approach. Such strategy may fit multifeedstock biodiesel plant needs but cannot be accepted for oils converted individually into biodiesel, because biodiesel yield will most likely fail, increasing costs. Transesterification optimization results depend on the chemical composition of vegetable oils and fats. Considering "sustainable" vegetable oils, biodiesel from Calophyllum inophyllum, Azadirachta indica, Terminalia catappa, Madhuca indica, Pongamia pinnata, and Jatropha curcas oils fits both current biodiesel standards: European EN 14214 and US ASTM D 6751 02. However, none of them can be considered to be the "ideal" alternative that matches all the main important fuel properties that ensure the best diesel engine behavior. In search of the ideal biodiesel composition, high presence of monounsaturated fatty acids (as oleic and palmitoleic acids), reduced presence of polyunsaturated acids, and controlled saturated acids content are recommended. In this sense, C18:1 and C16:1 are the best-fitting acids in terms of oxidative stability and cold weather behavior, among many other properties. Furthermore, genetic engineering is an invaluable tool to design oils presenting the most suitable fatty acid profile to provide high quality biodiesel. Finally, most published research related to engine performance and emissions fails in using a standard methodology, which should be implemented to allow the comparison between tests and biofuels from different origin. In conclusion, a compromise between social, economical, and technical agents must be reached.

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Biofuel characterization constitutes a substantial improvement in the valorization of this resource and allows a rational and controlled use of its energy potential. Corrosion and slagging are primary concerns of solid biofuels; the occurrence and extent of these phenomena depend significantly on the concentration of chlorine in the solid biofuel and the presence of elements such as potassium, sulfur, sodium, phosphorus, calcium, magnesium, iron, or silicon. Solid biofuels quality parameters are being determined by official methods established by the European Standard Technology Committee. Nevertheless, their implementation in the bioenergetic industry is scarce because these methods are expensive, tedious, and time-consuming. Therefore, a faster, more reliable, and cheaper analytical technique is mandatory in order to detect high concentrations of these parameters and avoid subsequent damages in heat exchanging surfaces. Near-infrared (NIR) spectroscopy is an eligible technique due to its high response speed, low cost per sample, absence of sample preparation, and versatility for the analysis of many different products and parameters. In this work, 250 samples of olive stone, olive tree pruning and dry depleted olive pomace, known also as "orujillo", have been collected, and NIR prediction model for determination of parameters such as chlorine, sulfur, and potassium have been obtained and evaluated. Correlation between actual and predicted values (R 2 ) was used to test the performance of calibrations. Practical utility of the validation models were assessed using the ratio of standard error of prediction to standard deviation of the reference data (RPD). High accuracy in prediction for a test set has been achieved for chlorine, sulfur, and potassium content (R 2 > 0.9 and RPD > 3), and standard error of prediction (SEP) values obtained in external validation with 53 independent samples are 129 mg kg −1 and 0.008% for chlorine and sulfur, respectively. This study illustrates the possibility of implementing the NIR technique in combination with multivariate data analysis to predict corrosive elements from olive byproducts in an economical and fast way.

To date, to simulate biodiesel production, kinetic models from different authors have been provided, each one usually applied to the use of a specific vegetable oil and experimental conditions. Models, which may include esterification, besides transesterification simulation, were validated with their own experimental conditions and raw material. Moreover, information about the intermediate reaction steps, besides catalyst concentration variation, is either rare or nonexistent. Here, in this work, a universal mathematical model comprising the chemical kinetics of a two-step (esterification and transesterification) vegetable oil-based biodiesel reaction is proposed. The proposed model is universal, as it may simulate any vegetable oil biodiesel reaction from the literature. For this purpose, a mathematical model using the software MATLAB has been designed. Using the mathematical model, the estimation of mass variation with time, of both reactants and products, as well as glyceride conversion and homogeneous catalyst concentration variation (instead of only alcohol/catalyst solution) are allowed. Moreover, analysis of the influence of some important variables affecting the reaction kinetics of biodiesel production (e.g., catalyst concentration), along with comparison and model validation with data from different authors may be carried out. In addition, Supplementary material with a collection of 290 rate constants, derived from 55 different experiments using different vegetable oils and conditions is provided.