A review of the feedstocks, catalysts, and intensification techniques for sustainable biodiesel production

Due to the dwindling crude oil resources and the detrimental environmental effects of toxic emissions from conventional diesel, biodiesel has gained importance over the last few years as a clean, sustainable, and renewable energy resource. As projected by the Energy Outlook review, a blend of 30 % biodiesel with petro-diesel (B30), is expected to be promoted as an alternative to petro-diesel by the year 2030. From the global perception, biodiesel is estimated to form 70 % of transport fuel which will be in demand by the year 2040. Since biodiesel is produced exclusively by vegetable oil, animal fat, microbial oil, and waste oil, it is renewable and biodegradable. Even though it has various benefits over conventional diesel fuel, yet numerous technical challenges need to be resolved. Among them are the cost of oil, selection of appropriate catalysts, and the development of a suitable technique for economical and efficient synthesis of biodiesel fuel from the vast range of available feedstocks. As 60-80 % cost of biodiesel is due to its feedstock, selection of low cost and sustainable feedstock from the vast varieties of available sources is the most challenging task to be achieved. In this article, the literature of the last twenty years was thoroughly studied and reviewed to examine the status of biodiesel. The present paper discusses the availability of different biodiesel feedstocks and summarises how their characteristics affect biodiesel properties along with the criteria of their selection for sustainable and low-cost production of biodiesel. The transesterification route is considered the most appropriate way for processing triglycerides of lipids into biodiesel which can take place with or without a catalyst. Different catalysts can homogeneously or heterogeneously catalyze the transesterification reaction depending on their solubility in the reaction mixture. The paper reviews the research on biodiesel production with different types of catalysts (alkaline, acidic, and enzymatic), their properties, preferred operating conditions, their suitability, and limitations. Work done on some of the latest intensification techniques for biodiesel production that are capable of managing the mass transfer restrictions of oil and alcohol phases namely, microwave irradiation, ultrasonication, and the co-solvent method are also reviewed. Some other intensification techniques that reduce biodiesel production cost by efficient separation, mixing, purification, and reaction like membrane reactor, motionless mixer, reactive distillation, and in-situ method have also been discussed along with their scope for commercialization in the context of the research work done till date. The review concludes with future perspectives and suggestions regarding the selection criteria of feedstocks, catalysts, and production techniques to make the production of biodiesel cost-effective, fast, energy-efficient, more cleaner, less complex, and sustainable.