INTRODUCTION
21st century has been facing many problems like energy sustainability, environmental problems and rising fuel prices. Conventional fuels are known for polluting air by emissions of sulfur dioxides, carbondioxides, particulate matter and other gases. This has resulted to increased research in alternate fuels and renewable source of energy .Moreover, energy consumption of the world is ever increasing, this has caused the fuel resources dwindle. The transport sector worldwide has considerably increased the fuel consumption reaching 61.5% of the total, especially in the last decade.Recent research expects that the amount of petrol in the world can be used merely for next 46 years. Hence, interest in research for an effective substitute for petroleum diesel is increasing . Currently , India produces only 30% of the total petroleum fuels required for its consumption and the remaining 70% is imported, which costs about Rs. 80,0000 million per year. It is evident that mixing of5% of biodiesel fuel to the present diesel fuel can save Rs.40, 000 million per year . – Over last few years, Biodiesel(fatty acid methyl esters) has become the part of the equation in the 1990’s as the effects of global warming began to get political acknowledgement, because of its benefits over petroleum diesel like significant reduction in greenhouse gas emissions , non-sulfur emissions and non-particulate matter pollutants, low toxicity, biodegradable and is obtained from renewable source like vegetable oils, animal fat etc.Biodiesel is superior to fossil diesel fuel in terms of exhaust emissions, cetane number, flash point and lubricity characteristics, without any significant difference in heat of combustion of these fuels . Moreover, biodiesel returns about 90% more energy than the energy that is utilized to produce it. Biodiesel mixed with conventional diesel in some proportions can be used to run any existing conventional compression ignition engine and does not require any modifications to be done to the engine .Due to benefits like renewable in nature, low cost and green house gas reduction potential, biodiesel is nowadays incorporated all over the world especially in developed countries like USA, France, Brazil in different proportions with diesel. It is also estimated that India can supplement 41.14% of its total diesel fuel consumption, if resources like waste cooking oil and other bio wastes were used as raw material for biodiesel production .
Biodiesel can be processed from different mechanisms. Some of them are 1. Direct use or blending in diesel fuel, 2. Micro emulsions in diesel fuel, 3. Thermal cracking of vegetable oils and 4. Transesterification. In this review, transesterification is focused.
Waste cooking oil refers to the used vegetable oil obtained from cooking food. Repeated frying for preparation of food makes the edible vegetable oil no longer suitable for consumption due to high free fatty acid (FFA) content [10]. Waste oil has many disposal problems like water and soil pollution, human health concern and disturbance to the aquatic ecosystem , so rather than disposing it and harming the environment, it can be used as an effective and cost efficient feedstock for Biodiesel production as it is readily available. Furthermore, Animal fats with high acid value and fat-containing floating sludge discharged in water systems are subject to environmental concern due to their high pollutant potential and it is a challenge for wastewater treatment plants to purify it. Therefore, conversion of low quality lipid-rich sources from slaughterhouses into commercial grade biodiesel is an opportune strategy for minimizing environmental damages while it can help meeting the energetic challenge. WCO collected can also be used to prepare soaps and additive for lubricating oil .Many researchers have successfully converted used vegetable oil into biodiesel. Vegetable oil contains saturated hydrocarbons (triglycerides) which consist of glycerol and esters of fatty acids . Used vegetable oil(UVO) is a by-product from hotels, fast food restaurants and shops selling fritter and by-product of an operating vegetable oil refinery.For serving better quality food, they usually throw this waste cooking without any treatment . In some places, UCO from restaurants were re-used by street sellers to fry their food ,this waste oil is termed as second –used cooking oil can also be utilized by converting to biodiesel.Distillate that is produced by deodorization of palm oil (DDPO) is also a promising and cost effective feedstock. Infact, using waste vegetable oil reduces the need for biodiesel-producing crops and the competition with food. UCOs have different properties from those of refined and crude vegetable oils .The chemical and physical properties of WCO are different from those of fresh oil since some changes due to chemical reactions – such as hydrolysis, oxidation, polymerization, and material transfer between food and vegetable oil occur during the frying process .The typical chemical and physical characteristics of WCO are shown in Table 1 .The usual values for Properties like density, kinematic viscosity, saponification value, acid value and Iodine value are shown in the table.