Sulfur Content

The sulfur content in biodiesel is very much less than that in fossil fuel. Due to the diversity of the feedstocks, there is variance in their sulfur contents. The sulfur content in biodiesel should be below 15 ppm. As per the ASTM D5373 standard, elemental analysis of carbon, nitrogen, hydrogen, sulfur, and oxygen can be done using CHNS analysis. The Flash Smart Elemental Analyzer can be used in conjunction with the Flame Photometric Detector (Thermoscientific) for sulfur determination in the fuel samples. The fuel samples are weighed in a hard tin container. The combustion process produces gases which were driven by the helium flow to a copper-filled layer, which is then sent through a water trap column, a short gas chromatography (GC) column. Finally, the determination of sulfur is made by the Flame Photometric Detector. The running time for sulfur determination is 5 minutes. The sulfur content in biodiesel is less than that in diesel. The amount of sulfur in diesel is 449.65 mg/kg, whereas the amounts of sulfur in palm methyl ester and Calophyllum inophyllum methyl ester are 3.82 mg/kg and 9.76 mg/kg, respectively. Jatropha biodiesel has 20 mg/kg sulfur content, which is the allowable limit according to standards (Liu et al. 2019).

Metal Content

Feedstocks gain their metal contents from soil and water during their living period. Similarly, during the production and storage of biodiesel, there is a chance of metals getting into the composition of the biodiesel (de Oliveira et al. 2009). The presence of metallic elements like Na, K, Ca, and Mg are monitored in biodiesel because the presence of these elements even at low concentrations can damage engine components, decrease performance, reduce the oxidation stability of biodiesel, and causes corrosion problems, and lead to contamination of fuel over storage or during usage (De Souza, Leocadio, and Da Silveira 2008).

The presence of sodium and potassium in biodiesel should be monitored because the catalysts used are their peroxides. These elements can exist as abrasive solids and insoluble soaps, leading to destruction and deterioration of engine parts (Chaves et al. 2008). Elements of calcium and magnesium may get into the biodiesel during the washing process, if hard water was employed or through contamination from the adsorbents used in cleaning process. These elements can lead to soap formation of different undesirable compounds in engines (Bondioli 2007). Atomic absorption spectrometry (AAS) is the most commonly used technique to find metal contents in biodiesel. Recently, flame AAS has been a widely adopted technique because it is capable of measuring the most important organometallic compounds in fuel (Lyra et al. 2010).

The PerkinElmer- PinAAcle 900T instrument for the study of atomic absorption spectrometry can be used for the determination of metal content. A 2-g sample was taken in the beaker, dissolved with a mixture of 5 mL H2S04 (96% w/v concentration) and 5 mL of HNO, (60% v/v concentration), and heated for the elimination of nitrous vapor. After completing this, a new mixture of 5 mL of HNO, and 5 mL of H2S04 was added to the digested product. The final solution was diluted in distilled water in a 50 mL volumetric flask. The sample was heated for 4 h daily at 80°C for 9 days to ensure complete digestion of the products. In AAS, analysis was done by aspiration (air/acetylene flame) for finding out the metal content in the product. The metal contents are determined with wavelengths of 248.3 nm, 357.87 nm, 279.48 nm, 213 nm, 217 nm, and 327.40 nm for Fe, Cr, Mn, Zn, Pb, and Cu, respectively (Santhoshkumar and Ramanathan 2020).

The presence of alkaline and alkaline earth ions (Na, K, Ca, and Mg) can result in residual deposits which obstruct the fuel injection system (Almeida et al. 2014). In this report, both the combined amounts of Na + К and Ca + Mg contained in Jatropha biodiesel complied with the requirements provided in ASTM D 6751 and EN14214 (almost 5 mg/kg). It was found that the combined amount of Na and К in biodiesel derived from Jatropha was 1.9 mg/kg, which suggested adequate washing out of the biodiesel trace catalyst of KOH. The combined amount of Ca and Mg in biodiesel derived from Jatropha, on the other hand, was 3.9 mg/kg. The total volume of Ca and Mg was greater than that of Na and K, which may be attributed to the characteristics in the raw materials like the natural presence of Ca and Mg and them remaining in the finished product (Tan et al. 2019).

Methanol Content

The residual methanol content present in the biodiesel should be monitored because even a small amount of methanol can decrease the flash point of biodiesel. The presence of residual methanol can affect engine parts like seals, fuel pumps, and elastomers, which can cause poor combustion characteristics. As per the EN I4l Ю standard, determination of residue methanol content was done using GC method. The GC system uses a megapore capillary column and a flame-ionizing detector with integrated computer software. Nitrogen gas was sent in at a flow of 3 mL/min. The detector temperature and injector port temperature were maintained at 210°C and 280°C, respectively. A splitless injection system was used for injecting the fuel and to avoid contamination between the samples (before every experiment of this type, the syringes should be cleaned, heated, and rinsed with distilled water). The oven was initially maintained at 38°C for 3 min and was raised to 250°C at the rate of 50°C/min; then it was maintained for 1 min w'ith the help of a computer program. Silitonga et al. found that excess methanol present in the methyl ester of palm oil and Calophyllum inophyllum was at levels of 0.24% and 7.18% in the yield which includes glycerin and other impurities (Silitonga et al. 2016).

Phosphorus Content

Crude plant oils have various impurities like acylglycerols, phospholipids, sugar, steroid, free fatty acids, and trace metals (Lin, Rhee, and Koseoglu 1997; AOCS 2009; Fan, Burton, and Austic 2010). Crude oil may have phosphorus content above 100 mg/kg-1 (Korn et al. 2007). According to the standard EN14214, the presence of phosphorus in biodiesel is regulated to less than 10 ppm. The presence of phosphorus is due to the oil-refining quality. Phospholipids contain glycerol with two fatty acids; this glycerol is attached to a phosphoric molecule (Mendow et al. 2011). In biodiesel, the presence of phosphorus content can damage the catalytic convertor in the exhaust line in diesel engines, which in turn can generate several pollutants such as particulate matter, carbon monoxide, and sulfur dioxide (Mittelbach 1996; Munack 2005).

The phosphorus content in the biodiesel is primarily removed by water degum- ming, and Emanuel et al. (2018) reported that usage of phosphoric acid can help to achieve better fuel properties than - raw Crambe abyssinica seed oil (Costa et al. 2018). According to the EN14214 standards, the phosphorus content in diesel can be determined by an atomic absorption spectrometer method (Dos Santos et al. 2007; Lira et al. 2011). The amount of phosphorus content present in crude soybean oil is 226 ppm. The coconut has phosphorus content of approximately 500 ppm, and phosphorus content of 7.1 ppm was obtained for biodiesel, which can be further reduced to 4.1 ppm with further treatment (Mendow et al. 2011). The phosphorous content in Karanja biodiesel was found to be 5 ppm, which is within permissible limits.

Free Glycerol and Total Glycerin

Glycerol is a by-product obtained during the transesterification in the presence of a chemically catalyzed reaction mixture of fatty acids. It is hygroscopic and immiscible in nature with FAME, it settles as a layer in biodiesel, and decantation is done when the reaction is completed. Glycerin in trace amounts is obtained as a valuable by-product in production biodiesel with some traces of mono-, di-, and triglycerides formed as intermediates. The residuals of catalyst, methanol, glycerol, and mono-, di-, and triglycerides are removed by water washing of biodiesel. Water of thrice the volume of biodiesel was used for the removal of the glycerol. Glycerol and glycerin are common words of almost the same meaning, but w'ith subtle differences. Glycerol is a pure compound, but glycerin refers to commercial scales, irrespective of their purity. Glycerin is a co-product obtained during the production of soap from oils, fats, fatty esters, and fatty acids. Glycerin has very low solubility in methyl esters, so it is easily removable w'ith the help of water from biodiesel because of its higher density. As per the EN 1405/ ASTM D6584 standard, the GC method is used for the determination of free glycerol and total glycerin.

Excessive free glycerol fuel may lead to material incompatibility, engine deposits, and engine combustion problems. A biodiesel’s free glycerol content should not exceed 0.020 wt.% and the overall total glycerol content should be 0.240 wt.% according to the ASTM D6751 standard (Meenakshi and Shyamala 2015). The free glycerin and total glycerin of Karanja biodiesel were within the limit of 0.0064 wt.% and 0.082 wt.% (Harreh et al. 2018). The Jatropha methyl ester has free glycerol and total glycerol of 0.01 and 0.012, respectively (Jain and Sharma 2014).

Mono-, Di-, and Triglycerides

During the transesterification, three reversible reactions happen, in which 1 mole of fatty methyl esters is released in every step. During the first reaction, the triglyceride is reacted to form diglyceride; in the second step, the diglyceride gets converted to monoglyceride; and in the final step, it is converted to glycerol. Monoglycerides are fatty acid esters of the mono type of glycerol. They are formed through chemical processes, and during the degradation they are formed as intermediates between triglyceride and diglyceride. Diglyceride consists of two fatty acids which are esterified to the trihydric alcohol glycerol. To find the amount of mono-, di-, and triglycerides present in the biodiesel sample, a gas chromatograph with a flame ionization detector is used as per the EN 14105 standard. In a temperature-controlled oven, a high- resolution silica capillary house is installed. The maximum capacity of the oven is 22.6 L. Electronically pressure-controlled systems are used to control the gases in the gas chromatography. There are separate injectors and detectors for liquid and gaseous samples. A flame-ionizing detector is used for liquid sample analysis and a thermal conductivity detector is used for gaseous samples. The temperature range of the setup is 30°C to 500°C with a 1°C set point resolution of accuracy. The heating rate varies from 1 to 50°C/min, and it has 1 to 7 segments of temperature profile. Using a TR-FAME capillary column, the biodiesel analysis was made. Capillary tubes will be 10 m in length and of 0.22 mm inner diameter, and the thickness of the film will be 0.25 pm.

The Karanja biodiesel was has monoglyceride, diglyceride, and triglyceride contents of 2.63 wt.%, 0.78 wt.%, and 0.06 wt.%, respectively. The Karanja biodiesel has glyceride contents within the standard limits of EN 14214, and the maximum allowable contents of monoglyceride, diglyceride, triglyceride are <0.8, <0.2, and <0.2, respectively (Harreh et al. 2018). The Jatropha biodiesel has monoglyceride, diglyceride, and triglyceride contents of 0.1 wt.%, 0.0 wt.%, and 0.0 wt.% respectively. The relationship between triglyceride, diglyceride, monoglyceride, and free fatty acid weights and the test sample mass was used to find the amount of the glyceride fractions in Jatropha oil (Amalia Kartika et al. 2013).

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