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An Overview of Biodiesel Production

      The advent of the industrial revolution had many benefits such as increases in wealth of the average masses, upgrade in living standards, and vast improvements in production of goods (both in quality and in quantity), which reduced prices drastically. Technological advancements also occurred in the transport sector, which enabled ease in travel, while the use of coal and petroleum skyrocketed: an example of this would be the 20‐fold increase in coal imports between 1550 and 1700 in Newcastle, England. Consequently, a proportional increase in mining of these fossilized reserves had to be done as far as from the early nineteenth century. Since then, the energy demand per capita has increased manifold to the point where current consumption trends cannot be supported without exhausting the remaining global reserves – alternative energy sources must be sought. Additionally, large areas of forest land had been cleared for fuelwood, which served as the primary energy source for cooking and heating in rural households. Widespread deforestation led to a rapid rise in global temperature since less trees are available for climate modulation. Also, upon using wood and other fossilized sources as fuel, huge amounts of particulate matter, smoke, and other noxious gases (NO_Xs, SO_Xs, CO, and CO₂) are emitted, and thus their continued emission for the last few centuries has led to global warming, harmful impacts on terrestrial and aquatic life (through acid rain, aquatic pollution resulting in eutrophication), and changes in weather patterns, which has even impacted the overall health and life expectancy of humans (lung diseases caused by air pollution, water pollution leading to chronic diseases, etc.).

In order to combat or gradually reverse the effects of such a global situation where arable land and potable water are scarce, alternative energy sources that have no or negligible environmental impacts must be sought. Thus, renewable energy research over the last few decades has been steadily increasing and is now capable of changing an entire country’s energy consumption trend. A good example is Brazil, which runs entirely on “sustainable” fuels, having produced 26.1% (a staggering 26.72 billion liters) of the global ethanol being used as fuel in 2017, and many countries have tried to replicate the so‐called “Brazilian ethanol model.” Among the wide variety of renewable energy sources available, feedstock for biofuels such as biodiesel and bioethanol are limited to a few varieties. Vegetable oils (edible or nonedible) cannot be directly used in engines due to their incompatible physicochemical properties. This had been tested by Dr. Rudolph Diesel who used peanut oil for his internal combustion (IC) engine and reported many problems in required performance when run for extended durations. Thus, such oils are converted into esters that are the main component of biodiesel, a fuel suitable for use in diesel engines with minor modifications. To convert vegetable oils as well as other potential feedstock such as microalgal lipids, animal fats and greases, waste oils, and other miscellaneous sources, various approaches may be used with different conversion efficiencies. The most efficient conversion process, however, is transesterification, which may or may not be coupled with an esterification pretreatment stage depending on the free fatty acid content of the oil.

      For both esterification and transesterification, the reactants are the feedstock and an alcohol, which in the presence of a catalyst are converted into their esters, producing either water or glycerol as by‐products. Depending on the reaction conditions (based on the approach used), catalysts may not be required, although a multitude of catalysts have been developed and tested with varying degrees of efficiency. Such catalysts range from the simplest mineral acids, enzymes, or bases, which are added for achieving a homogeneous system and discarded with every use to simple heterogeneous catalysts that rely on solid metal oxides or the use of inert carbonaceous or siliceous biomass doped with the required catalytic groups (including transition metals) or enzymes, as well as nanocatalysts that have increased efficiency (when compared with inert microporous support‐based catalysts), while specially designed catalysts based on resin supports or metal organic frameworks have also been developed and can be very efficient but may be difficult to commercialize due to high development costs and unavoidable losses in each cycle of use. Strangely, processes such as supercritical fluid technology or superheated vapor technology can function reliably even without the use of catalysts, although the use of catalysts can augment the process, which may require a cost‐to‐benefit analysis before commercialization.

      The process of biodiesel commercialization does not simply end at its production, since there are many stages that need to be considered for downstream processing as well as the consideration for treatment of hazardous materials generated (such as biodiesel wastewater that contains spent catalyst or leached ions) and the recovery of spent alcohol and the valorization of generated glycerol. Additionally, the produced fuel must have an acceptably long shelf life, and since biodiesel is prone to auto‐oxidation (it contains high oxygen content that helps in reducing pollution due to complete fuel combustion), such additives are essential for storage. Such processes generally increase the cost of available fuel, which has made it necessary to consider these hurdles that are yet to be overcome before the complete utilization of biodiesel is feasible as an environment‐friendly and affordable alternative to petrodiesel.

      Editors: Samuel Lalthazuala Rokhum, Gopinath Halder, Kanokwan Ngaosuwan, Suttichai Assabumrungrat