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Renewable sources include plants and animal fats, which are the main components of biofuels. Biofuels are free from sulfur, aromatics, metals, and crude oil residues. Since biofuels are more lubricating than petroleum diesel fuel, they are nonflammable and extend the life of diesel engines. As a result of this study, the main chemical and physical properties of biofuels were investigated, including their lubricity, viscosity, calorific value, and cetane number, which indicate the quality of renewable fuels, and compared with the other. We examined and compared the combustion characteristics of various types of biofuels as an alternative fuel, as well as their emissions characteristics. Biodiesel and biodiesel blends are compared to mineral diesel, as well as their performance in CI engines in this study’s review. With modified combustion equipment, biodiesel fuels can potentially reduce air pollution in diesel engines and are a very good substitute for fossil fuels. There is a need for more research and technological development in order for biofuels to become economically viable. Biofuel/biodiesel research should therefore be supported with policies that make their prices competitive with other conventional sources of energy. In the current state of affairs, biofuels are more effective when used alongside other sources of energy.
*Address all correspondence to: manju.tanwar@organicrecycling.co.in
World Energy Resources 2013 reported that 82% of electricity in 2013 was generated from fossil fuels, 13% from renewables, and the rest from nuclear sources [1]. Hydroelectric, wind, and solar power generate large amounts of power, but oil reserves are diminishing and could disappear within a century [2], making it essential to find replacements for petroleum-based fuels. A large portion of petroleum-based products are consumed by the transportation sector, which demands approximately 38% [3]. Scientists are also searching for alternative fuel sources due to the high price of fossil fuels, harmful greenhouse gas emissions, and the diversity of energy production. However, diesel engines still contribute significantly to greenhouse gas emissions and adverse effects on human health, despite better fuel economy and lower car taxes. There has been some research into solar cars as a solution to this issue, but their high cost and inconsistency make them unsuitable for daily use. Fuel and lubricant alternatives have been sought as a result [4]. Alternative sources of energy, such as solar, can still be utilized in a variety of ways. During photosynthesis, solar rays are converted into stored energy within plant tissue, which helps the plant live and reproduce through seed production. A biofuel can be produced using this energy change phenomenon. For compression ignition engines, biofuel produced from renewable sources is considered the best alternative to mineral diesel fuel.
The transportation industry contributes significantly to a country’s socioeconomic growth and development. Individuals’ quality of life is measured by the ease of moving goods and services. Transportation services are affordably and safely provided by governments across jurisdictions. Over 90% of the fossil fuel products are consumed in the transportation sector [5, 6]. By 2030, on-road transport will consume 50% of total energy, and by 2050, 80% [7]. Transport sector energy consumption in 2015 included passenger vehicles (cars and bikes), buses, air, passenger rail, and airline freight and heavy trucks, light trucks, and marine transport consume 35% of the energy used in the transportation sector. By 2050, liquid biofuel consumption will increase to 652 billion liters, and biodiesel consumption in the transport sector will rise to 180 billion liters [8]. By 2035, there will be more than 2 billion cars on the road, and this number will rise to 2.5 billion by 2050 [9, 10]. Environmental consequences and costs will be unimaginable if these cars run on fossil fuels. Low-carbon transport systems include biofuels, hydrogen, and electric vehicles (EVs). The use of ICEs will remain important in most developing countries for some time to come, despite the fact that hydrogen and electric vehicles avoid land use and impact air quality [11].
Most of investigations have been done based on use of the fuels in the diesel engine without any modification. Based on the so many literatures and studies of previous researchers, the authors have attempted to review important research on biodiesel production processes, biodiesel physical and chemical properties, and its performance and emission characteristics as compression ignition engine’s fuels.
Because of global warming and increasing air pollution, alternative fuels are increasingly being used in IC engines. Among the alternatives, biodiesel fuels seem remarkably interesting. They will be produced during a renewable way and possess certain advantageous properties that give them the potential to lower pollutants and CO2 emissions from IC engines.
The review deals with the status of biodiesel fuels and tries to elaborate the future direction for more wider utilization and the possible roles of biodiesel fuels in attaining the far-reaching goal of low-carbon economy using sustainable energy resources.
There are several options for biomass-derived fuels production involving chemical, biological, and thermochemical processes. An extensive map for these pathways can be seen in Figure 1 [12]. Two of the most promising fuels appear to be biodiesel or synthetic fuels such, as Fischer-Tropsch diesel [13]. This is because other potential fuels, such as Ethanol, Methanol, and LPG, do not perform as well in modern engines. Within IC engines, there are biofuel options for Gasoline (SI) and Diesel (CI) engines. Biogas and primary alcohols are the main fuels for SI engine. Ethanol from sugarcane was a key player in Brazil in the 1970s following the oil crisis, unfortunately the recent low petrol prices have undermined the green transport program decreasing the advantages. The government has overhauled their economic policy to ensure that the mandatory mix of ethanol in petrol is increased by 2.5% and have reinstated a levy on fossil fuels. A lot of current research is aimed toward diesel alternatives, as CI engines boast better fuel economy and lower car tax than petrol engines. An extensive map of pathways for the creation of all biofuels can be seen below in Figure 1 taken from IRENE Transport sector summary charts, 2014 [12].
As shown in Figure 1, vegetable oils are a source of biomass, which has the advantages over other energy sources of not exhausting the soil or damaging the environment [6]. Crude vegetable oils can be run through modern engines though their viscosity, calorific value, and freezing point are inferior to diesel fuel.
In literature, there are many examples of fuel properties for biodiesel created by transesterification, which involves removing the glycerides and combining oil esters of vegetable oil with alcohol. This creates fuels made up of alkyl esters of long chain fatty acids, usually found to be fatty acid methyl esters (FAMEs). The major cost in its production is the enzymatic reaction, which, due to the environment-friendly and less energy-intensive nature, prefers enzymes over chemical catalysts [14]. Hwang et al. found that producing biodiesel from Waste Cooking Oil (WCO) through transesterification with methanol and a Sodium Methoxide (NaOCH3) catalyst actually reduced the production of Carbon Monoxide (CO), Hydrocarbons (HC), and Particulate Matter (PM) at low loads, compared to diesel at conventional operating conditions [15]. A more recent development in transesterification and hydrogenation is to create biofuels from algae lipids. Alga is interesting due to its high production rates, lipid content, and rapid growth cycles [16]. They also do not compete for land growth and have the ability to grow anywhere in water [17]. However, Viêgas et al. found that oxidative stability was lower than that of soybean biodiesel; though by working on Palladium and Nickel catalysts, an optimized final product was produced [18]. Another technique of producing fuel to be examined from Figure 1 is Pyrolysis. The thermochemical reaction takes place at high temperatures in the absence of oxygen. One way to do this of present interest in Literature is by microwave heating, as this process of warming through radiation decreases the energy lost, increasing efficiency and economy, when compared to conventional heating methods [19]. Krutof et al., looked at pyrolysis oils and fish oils and the possibilities within blending, which they found increased their fuels’ calorific value [20].
To be able to synchronize biomass-derived fuels with applications for which they would be suited, the fuels must first be characterized. This is due to the high potential of differentiation between the chemical products, with different methods of creation and a huge range variable affecting the composition immensely. To start you may organize the feedstocks for biofuels into four main categories [7] as seen in Tables 1 and 2. To further categorize biofuels, they can be organized according technology of their production, on which they are greatly dependent. These are enlisted in Table 3. First-generation biofuels are potentially required to be focused on food production commodities and as a result may not be sustainable [3, 7]. FAME (Fatty Acid Methyl Ester), the most common biofuel in Europe, from vegetable oil, is considered a first-generation biofuel as it is exclusively produced using transesterification technology [21]. However, second-generation advanced biofuels, produced from Fischer-Tropsch synthesis, in addition to hydrothermal, pyrolysis liquefaction, and alternative catalytic procedures, maybe sustainable for future societies with their advantages as eco-friendly fuels [3, 7, 23]. These aim to overcome the limitations of first-generation biofuels [21], but they are still deeply reliant on the price of feedstock. Previous studies show that the feedstock cost embodies 75–80% of the total production cost [8]. In addition, the sources of the feedstock vary country to country and according to environmental conditions. For example, soybean for North America, sunflower and rapeseed for Europe, palm for Southeast Asia, coconut for tropic and sub-tropic areas, etc. [4]. Therefore, choosing the source with the highest oil yield is detrimental to success of producing low-cost biodiesel [8].
| Category | Edible vegetable oil | Non-edible vegetable oil | Waste or recycled oil | Animal fats |
|---|---|---|---|---|
| Examples | Canola, soybean, peanut, sunflower, palm and coconut oil | Jatropha curcas, Calophyllum inophyllum, Moringa oleifera and Croton megalocarpus | Waste or recycled oils both edible and nonedible | Chicken fat, pork lard, beef tallow and poultry fat |
A table showing categories of biofuel feedstocks [7].
